Add SOUP_MESSAGE_CAN_REBUILD, for regeneratable streamed request bodies

Add SOUP_MESSAGE_CAN_REBUILD, for regeneratable streamed request bodies

Long ago, soup_message_body_set_accumulate() was made into a no-op on
request bodies, because otherwise there would be no body available to
send if the request was restarted. Add a SOUP_MESSAGE_CAN_REBUILD
flag, that indicates that the caller takes responsibility for handling
this, and properly discard chunks if that is set.

Also update chunk-test to test this, and (finally!) remove the FIXME
there, since we are now testing the !accumulate codepath again
properly.

https://bugzilla.gnome.org/show_bug.cgi?id=656650