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 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.
Update hackney_url.erl

Fix "exception error: bad argument" for hackney_url:fix_path(<<"/">>) or hackney_url:fix_path(<<>>);

Error was:
(wsg@alg)30> hackney_url:fix_path(<<"/">>).
(<0.2999.0>) call hackney_url:fix_path(<<"/">>)
(<0.2999.0>) call hackney_url:fix_path(<<>>)
(<0.2999.0>) exception_from {hackney_url,fix_path,1} {error,badarg}
(<0.2999.0>) exception_from {hackney_url,fix_path,1} {error,badarg}
** exception error: bad argument
     in function  binary:part/2
        called as binary:part(<<>>,{0,-1})
     in call from hackney_url:fix_path/1 (src/hackney_url.erl, line 306)
Merge pull request #16 from Dumbris/master

Fix "exception error: bad argument" for hackney_url:fix_path(<<"/">>)
Merge pull request #15 from mweibel/issue-14-fix-http-status-code-parsing

Fix #14: Parse incomplete HTTP status codes