defaultValue Introspection bug

[bruce]

@benwilson512 Since your 924248b change I've run into a similar error to the following in a Relay app trying to parse the schema.json (blocking all progress):

graphql/graphiql#89

Based on the final response, my best guess is that instead of serializing the default value as, eg "2016-04-26T06:20Z" it needs to be wrapped in quotes, eg: "\"2016-04-26T06:20Z\""