Set Content-Type header for ?format=TEXT content replies on paths

HTTP servers should always send a Content-Type response header to
prevent browsers from entering into (a possibly stupid) sniffing
mode. Since the payload is wrapped up in base64 this is really a
text/plain reply.

Using the two-arg form of startRenderText() will send a text/plain
Content-Type, which is exactly correct here.

Change-Id: I194a1446c62e4409c27e8fc72aa43846ea3438a9