slice plugin: bug with self formed 416 responses.

[traeak]

This bug showed up when self healing was added. Before the slice plugin would only generate 416 responses in extremely rare conditions. With the self healing it seems to now handle 416 generation itself. There's a bug where the handleFirstServerHeader function didn't properly exit after issuing the TSHttpHdrPrint function to the downstream TSIOBuffer, continuing to manipulate the header TSMBuffers.