Fix support for generator-based WSGI apps (#887)

Fix support for generator-based WSGI apps (#887)

* Handle generator WSGI app

* Lint code

* Add type annotations

* Add more tests

* Refactor test to use application_factory

* Remove content length as it's misleading

* Add test for WSGI generator

* Add test for empty generator

* Remove previous tests

* Move docstring to a comment

* Fix whitespace

* Fix name of function

Co-Authored-By: Florimond Manca <florimond.manca@gmail.com>
* Update tests/test_wsgi.py

Co-Authored-By: Florimond Manca <florimond.manca@gmail.com>
* Update tests/test_wsgi.py

Co-Authored-By: Florimond Manca <florimond.manca@gmail.com>
* Update httpx/_dispatch/wsgi.py

Co-Authored-By: Florimond Manca <florimond.manca@gmail.com>
Co-authored-by: Florimond Manca <florimond.manca@gmail.com>