Removed support for encoding BigDecimal as a JSON number

This is because the new encoder will no longer support encode_json.
Therefore our only choice is to return `to_i` or `to_s` in
`BigDecimal#as_json`. Since casting a BigDecimal to an integer is
most likely a lossy operation, we chose to encode it as a string.

Support for encoding BigDecimal as a string will return via the
`activesupport-json_encoder` gem.

activesupport/lib/active_support/core_ext/object/json.rb

@@ -132,15 +132,8 @@ class BigDecimal
   # if the other end knows by contract that the data is supposed to be a
   # BigDecimal, it still has the chance to post-process the string and get the
   # real value.
-  #
-  # Use <tt>ActiveSupport.encode_big_decimal_as_string = true</tt> to
-  # override this behavior.
   def as_json(options = nil) #:nodoc:
-    if finite?
-      ActiveSupport.encode_big_decimal_as_string ? to_s : self
-    else
-      nil
-    end
+    finite? ? to_s : nil
   end
 end
 

activesupport/lib/active_support/json/encoding.rb

@@ -7,7 +7,6 @@ module ActiveSupport
   class << self
     delegate :use_standard_json_time_format, :use_standard_json_time_format=,
       :escape_html_entities_in_json, :escape_html_entities_in_json=,
-      :encode_big_decimal_as_string, :encode_big_decimal_as_string=,
       :to => :'ActiveSupport::JSON::Encoding'
   end
 
@@ -69,10 +68,6 @@ class << self
         # to the Active Support legacy format.
         attr_accessor :use_standard_json_time_format
 
-        # If false, serializes BigDecimal objects as numeric instead of wrapping
-        # them in a string.
-        attr_accessor :encode_big_decimal_as_string
-
         attr_accessor :escape_regex
         attr_reader :escape_html_entities_in_json
 
@@ -118,7 +113,6 @@ def const_missing(name)
 
       self.use_standard_json_time_format = true
       self.escape_html_entities_in_json  = true
-      self.encode_big_decimal_as_string  = true
     end
   end
 end

activesupport/test/json/encoding_test.rb

@@ -399,17 +399,6 @@ def test_struct_encoding
                  ActiveSupport::JSON.decode(json_string_and_date))
   end
 
-  def test_opt_out_big_decimal_string_serialization
-    big_decimal = BigDecimal('2.5')
-
-    begin
-      ActiveSupport.encode_big_decimal_as_string = false
-      assert_equal big_decimal.to_s, big_decimal.to_json
-    ensure
-      ActiveSupport.encode_big_decimal_as_string = true
-    end
-  end
-
   def test_nil_true_and_false_represented_as_themselves
     assert_equal nil,   nil.as_json
     assert_equal true,  true.as_json