Improve ByteBuffer copy method

This commit improves JettyWebSocketHandlerAdapter::copyByteBuffer so
that it allocates a buffer large enough for the remaining bytes
contained in the source, instead of allocating one with the capacity of
the source.

Closes gh-31857

Author: poutsma

spring-websocket/src/main/java/org/springframework/web/socket/adapter/jetty/JettyWebSocketHandlerAdapter.java

@@ -118,8 +118,9 @@ public void onWebSocketFrame(Frame frame, Callback callback) {
 	}
 
 	private static ByteBuffer copyByteBuffer(ByteBuffer src) {
-		ByteBuffer dest = ByteBuffer.allocate(src.capacity());
-		dest.put(0, src, 0, src.remaining());
+		ByteBuffer dest = ByteBuffer.allocate(src.remaining());
+		dest.put(src);
+		dest.flip();
 		return dest;
 	}