Allow for 3xx range headers #249

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?