Do note remove `Content-Type` when `render :body`

`render :body` should just not set the `Content-Type` header. By
removing the header, it breaks the compatibility with other parts.

After this commit, `render :body` will returns `text/html` content type,
sets by default from `ActionDispatch::Response`, and it will preserve
the overridden content type if you override it.

Fixes #14197, #14238

This partially reverts commit 30473768.

actionpack/lib/action_controller/metal/rack_delegation.rb

@@ -5,8 +5,8 @@ module ActionController
   module RackDelegation
     extend ActiveSupport::Concern
 
-    delegate :headers, :status=, :location=, :content_type=, :no_content_type=,
-             :status, :location, :content_type, :no_content_type, :to => "@_response"
+    delegate :headers, :status=, :location=, :content_type=,
+             :status, :location, :content_type, :to => "@_response"
 
     def dispatch(action, request)
       set_response!(request)

actionpack/lib/action_controller/metal/rendering.rb

@@ -45,9 +45,7 @@ def _render_in_priorities(options)
     def _process_format(format, options = {})
       super
 
-      if options[:body]
-        self.headers.delete "Content-Type"
-      elsif options[:plain]
+      if options[:plain]
         self.content_type = Mime::TEXT
       else
         self.content_type ||= format.to_s

actionpack/lib/action_dispatch/http/response.rb

@@ -63,8 +63,6 @@ class Response
     # content you're giving them, so we need to send that along.
     attr_accessor :charset
 
-    attr_accessor :no_content_type # :nodoc:
-
     CONTENT_TYPE = "Content-Type".freeze
     SET_COOKIE   = "Set-Cookie".freeze
     LOCATION     = "Location".freeze
@@ -305,17 +303,8 @@ def append_charset?
       !@sending_file && @charset != false
     end
 
-    def remove_content_type!
-      headers.delete CONTENT_TYPE
-    end
-
     def rack_response(status, header)
-      if no_content_type
-        remove_content_type!
-      else
-        assign_default_content_type_and_charset!(header)
-      end
-
+      assign_default_content_type_and_charset!(header)
       handle_conditional_get!
 
       header[SET_COOKIE] = header[SET_COOKIE].join("\n") if header[SET_COOKIE].respond_to?(:join)

actionpack/test/controller/new_base/render_body_test.rb

@@ -65,6 +65,11 @@ def with_custom_layout
       render body: "hello world", layout: "greetings"
     end
 
+    def with_custom_content_type
+      response.headers['Content-Type'] = 'application/json'
+      render body: '["troll","face"]'
+    end
+
     def with_ivar_in_layout
       @ivar = "hello world"
       render body: "hello world", layout: "ivar"
@@ -141,6 +146,13 @@ class RenderBodyTest < Rack::TestCase
       assert_status 200
     end
 
+    test "specified content type should not be removed" do
+      get "/render_body/with_layout/with_custom_content_type"
+
+      assert_equal %w{ troll face }, JSON.parse(response.body)
+      assert_equal 'application/json', response.headers['Content-Type']
+    end
+
     test "rendering body with layout: false" do
       get "/render_body/with_layout/with_layout_false"
 
@@ -154,22 +166,5 @@ class RenderBodyTest < Rack::TestCase
       assert_body "hello world"
       assert_status 200
     end
-
-    test "rendering from minimal controller returns response with no content type" do
-      get "/render_body/minimal/index"
-
-      assert_header_no_content_type
-    end
-
-    test "rendering from normal controller returns response with no content type" do
-      get "/render_body/simple/index"
-
-      assert_header_no_content_type
-    end
-
-    def assert_header_no_content_type
-      assert_not response.headers.has_key?("Content-Type"),
-        %(Expect response not to have Content-Type header, got "#{response.headers["Content-Type"]}")
-    end
   end
 end

actionpack/test/dispatch/response_test.rb

@@ -235,14 +235,6 @@ def test_response_body_encoding
       assert_equal @response.body, body.each.to_a.join
     end
   end
-
-  test "does not add default content-type if Content-Type is none" do
-    resp = ActionDispatch::Response.new.tap { |response|
-      response.no_content_type = true
-    }
-
-    assert_not resp.headers.has_key?('Content-Type')
-  end
 end
 
 class ResponseIntegrationTest < ActionDispatch::IntegrationTest

actionview/lib/action_view/helpers/rendering_helper.rb

@@ -17,8 +17,9 @@ module RenderingHelper
       # * <tt>:html</tt> - Renders the html safe string passed in out, otherwise
       # performs html escape on the string first. Setting the content type as
       # <tt>text/html</tt>.
-      # * <tt>:body</tt> - Renders the text passed in, and does not set content
-      # type in the response.
+      # * <tt>:body</tt> - Renders the text passed in, and inherits the content
+      # type of <tt>text/html</tt> from <tt>ActionDispatch::Response</tt>
+      # object.
       #
       # If no options hash is passed or :update specified, the default is to render a partial and use the second parameter
       # as the locals hash.

actionview/lib/action_view/rendering.rb

@@ -102,11 +102,6 @@ def _render_template(options) #:nodoc:
       # Assign the rendered format to lookup context.
       def _process_format(format, options = {}) #:nodoc:
         super
-
-        if options[:body]
-          self.no_content_type = true
-        end
-
         lookup_context.formats = [format.to_sym]
         lookup_context.rendered_format = lookup_context.formats.first
       end

guides/source/layouts_and_rendering.md

@@ -304,10 +304,13 @@ type, by using the `:body` option to `render`:
 render body: "raw"
 ```
 
-TIP: This option should be used only if you explicitly want the content type to
-be unset. Using `:plain` or `:html` might be more appropriate in most of the
+TIP: This option should be used only if you don't care about the content type of
+the response. Using `:plain` or `:html` might be more appropriate in most of the
 time.
 
+NOTE: Unless overriden, your response returned from this render option will be
+`text/html`, as that is the default content type of Action Dispatch response.
+
 #### Options for `render`
 
 Calls to the `render` method generally accept four options: