Pull buffer assignment up

Since everything goes through a `run` method, we can pull the buffer
assignment up.

actionview/lib/action_view/base.rb

@@ -212,6 +212,7 @@ def initialize(context = nil, assigns = {}, controller = nil, formats = nil) #:n
 
     def run(method, locals, buffer, &block)
       _old_output_buffer = @output_buffer
+      @output_buffer = buffer
       send(method, locals, buffer, &block)
     ensure
       @output_buffer = _old_output_buffer

actionview/lib/action_view/template/handlers/erb/erubi.rb

@@ -13,7 +13,7 @@ def initialize(input, properties = {})
 
             # Dup properties so that we don't modify argument
             properties = Hash[properties]
-            properties[:preamble]   = "@output_buffer = output_buffer;"
+            properties[:preamble]   = ""
             properties[:postamble]  = "@output_buffer.to_s"
             properties[:bufvar]     = "@output_buffer"
             properties[:escapefunc] = ""