Add missing Content-Type header on JSON responses

[fabiante]

Hi there, thanks for the awesome package! I previously relied on using httpbin.org in tests but prefer having a local test server.

In some of my tests, i asserted that the response of some endpoints is status 200 and content type is "application/json".

Migrating to this package, these tests failed because the content type is not set.

As far as I am concerned this should be fine, right?

Sincerely!