Only prepend http:// to URI if it lacks a scheme.

Previously rest-client would add `http://` to URIs even if they had a
scheme, so `ftp://example.com` would become `http://ftp//example.com`.

Instead, only prepend `http://` to URIs that do not start with a scheme
followed by `://`.

Also add tests for this behavior, since my first attempt at this
functionality would have broken URIs with a port and no scheme.

Fixes #299