Simplify logic of REST request suffix parsing.

This patch changes the way the suffix (giving the requested data format) is
parsed for REST requests. Before, the string was split at '.'
characters and it was assumed that the second part was the suffix.

Now, we look for the last dot and use that to determine the suffix.
This allows for strings that contain dots (not used now, though), and
seems, in general, to be clearer and more intuitive.