Properly quote json keys.

According to the RFC and the json.org site all json keys must be strings, and those strings must be quoted with double quotes.
[#1755 state:committed]

activesupport/lib/active_support/json/encoders/hash.rb

@@ -5,7 +5,7 @@ class Hash
   # the hash keys. For example:
   #
   #   { :name => "Konata Izumi", 'age' => 16, 1 => 2 }.to_json
-  #   # => {"name": "Konata Izumi", 1: 2, "age": 16}
+  #   # => {"name": "Konata Izumi", "1": 2, "age": 16}
   #
   # The keys in the JSON string are unordered due to the nature of hashes.
   #
@@ -39,7 +39,7 @@ def to_json(options = {}) #:nodoc:
 
     returning result = '{' do
       result << hash_keys.map do |key|
-        "#{ActiveSupport::JSON.encode(key)}: #{ActiveSupport::JSON.encode(self[key], options)}"
+        "#{ActiveSupport::JSON.encode(key.to_s)}: #{ActiveSupport::JSON.encode(self[key], options)}"
       end * ', '
       result << '}'
     end

activesupport/test/json/encoding_test.rb

@@ -59,7 +59,7 @@ def test_hash_encoding
     assert_equal %({\"a\": \"b\"}), { :a => :b }.to_json
     assert_equal %({\"a\": 1}), { 'a' => 1  }.to_json
     assert_equal %({\"a\": [1, 2]}), { 'a' => [1,2] }.to_json
-    assert_equal %({1: 2}), { 1 => 2 }.to_json
+    assert_equal %({"1": 2}), { 1 => 2 }.to_json
 
     sorted_json = '{' + {:a => :b, :c => :d}.to_json[1..-2].split(', ').sort.join(', ') + '}'
     assert_equal %({\"a\": \"b\", \"c\": \"d\"}), sorted_json
@@ -80,7 +80,7 @@ def test_exception_raised_when_encoding_circular_reference
 
   def test_hash_key_identifiers_are_always_quoted
     values = {0 => 0, 1 => 1, :_ => :_, "$" => "$", "a" => "a", :A => :A, :A0 => :A0, "A0B" => "A0B"}
-    assert_equal %w( "$" "A" "A0" "A0B" "_" "a" 0 1 ), object_keys(values.to_json)
+    assert_equal %w( "$" "A" "A0" "A0B" "_" "a" "0" "1" ).sort, object_keys(values.to_json)
   end
 
   def test_hash_should_allow_key_filtering_with_only