PocoRestRequestHandler: fix behaviour when caching is negative
PocoRestRequestHandler: fix behaviour when caching is negative

When the RestAction::caching() property has a negative value then no
caching headers should be injected.

Fixes: 5a2bc7dd837f ("PocoRestRequestHandler: simple HTTP caching capabilities")
Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server