Fix #14: Parse incomplete HTTP status codes

The fix works but the changes needed for testing it
are probably not what you want. I suppose in order to
prevent any changes in hackney_http, I'd need to do the
tests inline but as I didn't see this in the other source code I decided
to make an outside test working.

However I think you might not like these changes, therefore
please guide me what would be the best option.

Also I think there might be a better naming for the case variables. If you have some
suggestions, please tell me.

(This fixes #14 - apparently github doesn't parse subject lines in PRs, so I added this line to add the link)