Return raw body for UrlEncoded requests.

Return raw body for UrlEncoded requests.

This is for RT #111876:
https://rt.cpan.org/Public/Bug/Display.html?id=111876

We currently return a body filehandle for OctetStream requests, but not for
UrlEncoded requests, which causes surprises - for e.g. Dancer1 issue:
https://github.com/PerlDancer/Dancer/issues/1140

This change makes parsed UrlEncoded requests store the "raw" body (after parsing
out chunking etc) in $self->body, so the caller can get a filehandle for it just
like they would for an OctetStream request.

Tests updated to expect this new behaviour.

TODO: Decide if we need MultiPart to expose a raw body for consistency (would
anyone ever want the overall raw body for a MultiPart request?) and if so,
cobble together the request as it was (but not storing the form params part to
disk for security)