Add test for json_name functionality

ruby/tests/encode_decode_test.rb

diff --git a/ruby/tests/encode_decode_test.rb b/ruby/tests/encode_decode_test.rb
index cce364d..429ac43 100755
--- a/ruby/tests/encode_decode_test.rb
+++ b/ruby/tests/encode_decode_test.rb
@@ -95,4 +95,10 @@
     end
   end
 
+  def test_json_name
+    msg = A::B::C::TestJsonName.new(:value => 42)
+    json = msg.to_json
+    assert_match json, "{\"CustomJsonName\":42}"
+  end
+
 end

ruby/tests/generated_code.proto

diff --git a/ruby/tests/generated_code.proto b/ruby/tests/generated_code.proto
index e811669..bfdfa5a 100644
--- a/ruby/tests/generated_code.proto
+++ b/ruby/tests/generated_code.proto
@@ -83,3 +83,7 @@
   map<string, TestUnknown> map_unknown = 67;
   int32 unknown_field = 89;
 }
+
+message TestJsonName {
+  int32 value = 1 [json_name = "CustomJsonName"];
+}