The reference for a request verb is now the string which is used
everywhere else, instead of the symbol corresponding to it. This was an
artifact from the import from httprb, and there is no advantage in it,
since these strings are frozen in most use cases, and the
transformations from symbol to strings being performed everywhere are
prooof that keeping the atom isn't really bringing any benefit.
* `Response#error`, which, coupled with `ErrorResponse#error`, allows
for `if response.error` kind of conditional;
* `Response#raise_for_status` now returns the response when no error is
raise (for method chaining);
Closes#153
replacing them with SimpleDelegator where it made sense, and removing
them altogether whenever possible. ErrorResponse doesn't have a special
handling for Response methods anymore, but it's highly debatable if it
should.
