content type method added to core requests
- CoreGsonRequest extended from JsonRequest instead of Request
- methods implemented by JsonRequest are removed from CoreGsonRequest
- Gson object is now instantiated inside CoreGsonRequest since CoreBaseRequest has nothing to do with it
- getHeaders method now checks if there are duplicate Content-Type header definitions and throws if any
- copyright statement indentation
- now unused contentType method of CoreBaseRequest removed
readme update to indicate that json is the only supported content-type
Merge pull request #7 from zeplin/feature/content-type

Add content type method to core requests