Unicode - do not warn for legacy apps

Lets skip the decoding process (like we do for encoding in
finalize_headers) for apps not using core encoding support,
that is, when "encoding" config bit is undef.
Unicode - fix decoding process for uploads

[error] Caught exception in engine "Can't use string ("Can't call method
"decode" on an"...) as a HASH ref while "strict refs" in use at
Catalyst/Plugin/Unicode/Encoding.pm line 101."
Revert "Unicode plugin - rework exception handler"

This reverts commit 4fb27043578a31f1ffd7498d1097c817a932d173.

No longer need to warn instead of die for failed decoding/encoding
process due 95c85a0e3248f73938388889b599c3ddf67ac4fd.