Added fr::Socket::has_data to check if recv buffer is empty

If using a SocketSelector, you should keep accepting data from the socket until has_data returns false, indicating that you've read everything.

src/SSLSocket.cpp

@@ -168,6 +168,11 @@ namespace fr
         is_connected = true;
         ssl_socket_descriptor = std::move(context);
     }
+
+    bool SSLSocket::has_data() const
+    {
+        return !unprocessed_buffer.empty();
+    }
 }
 
 #endif