Various fixes and updates

Separated getters and setters for the Response object, i.e. `getBody` and `setBody` instead of just `body`.

Accessing errors and routes is now more verbose, e.g. `getErrors` instead of `errors`.

Write function added to the Response class; will be used later by filters et.c.

Nicer class property documentation and proper getter-methods for Request properties.

No more lowercasing of keys in Blobs.

Proper test for offsetExists.

Build passes on `>=5.3`.