Allow for 3xx range headers

Everything above 299 is considered an error, which is plainly incorrect.

To get around this I had to:

1. add an error decoder
2. throw a special exception for 3xx range
3. when catching runtime errors, check if it is the special 300 error
4. convert to a valid response entity

Will investigate replacing this line:

if (response.status() >= 200 && response.status() < 300) {
in SynchronousMethodHandler and PR'ing it. I just wonder why nobody else has come across this issue.

Should I fix it?

Start in SynchronousMethodHandler at the response-status condition quoted in the issue and inspect how 3xx responses are decoded and converted to response entities. Add or update coverage for 3xx responses with headers, and consider the issue done when those responses no longer enter the error path while other status handling remains unchanged.