url: handle scheme omission

When the scheme is omitted in URL overriding (for example `example.com`
instead of `https://example.com`), s3fs is modifying the URL by
inserting `s3.` in the middle of the name  (`examples3..com`).

This can be a bit difficult to troubleshoot and curl seems to handle
schema-less requests just fine. So, just handle this case correctly.
Merge pull request #74 from vincentbernat/fix/url-may-omit-scheme

url: handle scheme omission