Fix lifetime on send()

Fixes drogue-iot#78

src/client.rs

@@ -282,7 +282,7 @@ where
     ///
     /// The response is returned.
     pub async fn send<'req, 'buf, B: RequestBody>(
-        &'req mut self,
+        &'conn mut self,
         request: Request<'req, B>,
         rx_buf: &'buf mut [u8],
     ) -> Result<Response<'req, 'buf, HttpConnection<'conn, T>>, Error> {