examples/psycopg2_pool.py: make it possible to pass arguments to cursor() in execute() and fetch*() methods

pywsgi: raise an AssertionError if non-zero content-length is passed to start_response(204 or 304) or if non-empty body is attempted to be written for 304/204 response

also pywsgi no longer capitalizes headers for you

otherwise we're compiling the extensions with header files that do not
match the libraries being linked against.

resolver_thread: set expected_errors to Exception because it is possible for DNS functions to raise UnicodeDecodeError