Encrypt responses in addition to decrypting requests

Currently, fernet-rack only decrypts Fernet-encoded requests; it does not do anything about responses. This extends the mangling to include responses as well.

Additionally, it fixes some test issues: CONTENT_TYPE must be passed to `Rack::MockRequest#get` as a String key, and the minitest assertion format is `assert_equal expected, actual` (the juxtaposed variant also works, of course, but leads to confusing error messages).
Maybe naming that ``read_body`` ?