Post-release version bump to 0.2.2a
Set content type to application/json for both success and error responses
Move content setting to .write_* methods on BaseHandler.
Add tests for content type on all .write_* methods.
Merge branch 'feature/issue-23-set-response-content-type-to-json' into develop
Merge branch 'develop' into release/0.2.x
Version 0.2.2