Update Conduit to psr/http-message 0.5.1 and phly/http 0.7.0

Updates Conduit to use psr/http-message 0.5.1 and phly/http 0.7.0. Changes include:

- The HTTP decorators have been updated to follow `Psr\Http\Message\IncomingRequestInterface` changes and `Psr\Http\Message\OutgoingResponseInterface`. In some cases, that means the
 removal of setters, and changes to constructors.
- URLs are now handled internally by the HTTP messages as strings; whenever property access was required, the URI is either cast to a `Phly\Http\Uri` instance or `parse_url()` is used.
- Tests were updated to accommodate the changes to `IncomingRequestInterface` (primarily around the constructor).