LibHTTP: Relax the finish_up() "must be called once" limitation a bit

It's alright for this function to be called multiple times, as it quits
early when a partial flush doesn't empty the download buffer.
Relax the assertion to having scheduled "did_finish()" only once.

Userland/Libraries/LibHTTP/Job.h

@@ -64,6 +64,7 @@ protected:
     Optional<size_t> m_current_chunk_total_size;
     bool m_can_stream_response { true };
     bool m_should_read_chunk_ending_line { false };
+    bool m_has_scheduled_finish { false };
 };
 
 }