Fix uri generation when provided relative path

Summary:
If setPath is called on a uri object like
`$uri = id(new PhutilURI('foo.example'))->setPath('bar')`, the uri generated
by the __toString method does not put a slash between the domain and the path,
resulting in `foo.examplebar` instead of `foo.example/bar`.

Test Plan: arc unit

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5527