Add flush method to ensure frame is correctly written (#88)

When the underlying stream is buffered (i.e: TlsStream<TcpStream>)
calling write_frame is not enough to be guaranteed that data is pushed
down to the last stream (i.e: on the network in case of
TlsStream<TcpStream>).

Currently there is no way to force flush the data on the stream, so
write_frame call may never push the data.
https://docs.rs/tokio-rustls/latest/tokio_rustls/index.html#why-do-i-need-to-call-poll_flush

The commit introduce a flush method, to allow the user of the lib to
ensure all data is correctly pushed down.

src/lib.rs

@@ -342,6 +342,21 @@ impl<'f, S> WebSocketWrite<S> {
   {
     self.write_half.write_frame(&mut self.stream, frame).await
   }
+
+  pub async fn flush(&mut self) -> Result<(), WebSocketError>
+  where
+    S: AsyncWrite + Unpin,
+  {
+    flush(&mut self.stream).await
+  }
+}
+
+#[inline]
+async fn flush<S>(stream: &mut S) -> Result<(), WebSocketError>
+where
+  S: AsyncWrite + Unpin,
+{
+  stream.flush().await.map_err(|e| WebSocketError::IoError(e))
 }
 
 /// WebSocket protocol implementation over an async stream.
@@ -495,6 +510,18 @@ impl<'f, S> WebSocket<S> {
     Ok(())
   }
 
+  /// Flushes the data from the underlying stream.
+  ///
+  /// if the underlying stream is buffered (i.e: TlsStream<TcpStream>), it is needed to call flush
+  /// to be sure that the written frame are correctly pushed down to the bottom stream/channel.
+  ///
+  pub async fn flush(&mut self) -> Result<(), WebSocketError>
+  where
+    S: AsyncWrite + Unpin,
+  {
+    flush(&mut self.stream).await
+  }
+
   /// Reads a frame from the stream.
   ///
   /// This method will unmask the frame payload. For fragmented frames, use `FragmentCollector::read_frame`.