Allow "0" (and any other falsy) URI paths

Some strings are not converted into proper URLs when Request objects are created through `createRequest`. For example, passing a `string(1) "0"` will fail because [this ternary conditional](https://github.com/guzzle/guzzle/blob/master/src/Client.php#L186) isn't assigning it properly (it refuses to combine any false-like values). But there are a lot of use cases where the before mentioned strings are needed, say doing a `POST` on `http://foo.com/0`. I've added a test case, so hopefully that should explain what I mean.