* URIs can now be relative.

* `multipart/form-data` headers are now overridden case-insensitively.
* URI paths no longer encode the following characters because they are allowed
  in URIs: "(", ")", "*", "!", "'"
* A port is no longer added to a URI when the scheme is missing and no port is
  present.

Initial release.

Currently unsupported:

- `Psr\Http\Message\ServerRequestInterface`
- `Psr\Http\Message\UploadedFileInterface`