chore(lib): fix new unused_mut warnings in nightly

src/http/conn.rs

@@ -240,7 +240,7 @@ where I: AsyncRead + AsyncWrite,
 
         let wants_keep_alive = head.should_keep_alive();
         self.state.keep_alive &= wants_keep_alive;
-        let mut buf = self.io.write_buf_mut();
+        let buf = self.io.write_buf_mut();
         // if a 100-continue has started but not finished sending, tack the
         // remainder on to the start of the buffer.
         if let Writing::Continue(ref pending) = self.state.writing {