Fill request headers from the environ dictionary and
        modify them via the list of :attr:`headers_middleware`.
        The returned headers will be sent to the target uri.

def request_headers(self):
        '''Fill request headers from the environ dictionary and
        modify them via the list of :attr:`headers_middleware`.
        The returned headers will be sent to the target uri.
        '''
        headers = CIMultiDict()
        for k in self.environ:
            if k.startswith('HTTP_'):
                head = k[5:].replace('_', '-')
                headers[head] = self.environ[k]
        for head in ENVIRON_HEADERS:
            k = head.replace('-', '_').upper()
            v = self.environ.get(k)
            if v:
                headers[head] = v
        for middleware in self.wsgi.headers_middleware:
            middleware(self.environ, headers)
        return headers