Perf: speed up json encoding.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

activesupport/lib/active_support/json/encoding.rb

@@ -232,9 +232,8 @@ def as_json(options = nil) #:nodoc:
 
     # use encoder as a proxy to call as_json on all values in the subset, to protect from circular references
     encoder = options && options[:encoder] || ActiveSupport::JSON::Encoding::Encoder.new(options)
-    pairs = subset.map { |k, v| [k.to_s, encoder.as_json(v)] }
-    result = self.is_a?(ActiveSupport::OrderedHash) ? ActiveSupport::OrderedHash.new : Hash.new
-    pairs.inject(result) { |hash, pair| hash[pair.first] = pair.last; hash }
+    result = self.is_a?(ActiveSupport::OrderedHash) ? ActiveSupport::OrderedHash : Hash
+    result[subset.map { |k, v| [k.to_s, encoder.as_json(v)] }]
   end
 
   def encode_json(encoder)