strip non-UTF8 chars from POST params

before this change, POSTs containing params with non-UTF8 chars lead
to ArgumentError ("invalid byte sequence in UTF-8") and a 500 error.
Such POSTs can be crafted using exploit scripts.

The implementation of the sanitisation is borrowed fro the utf8-cleaner
gem (https://github.com/singlebrook/utf8-cleaner).
add more explanation, small bugfix to UTF8Cleaner
UTF8Cleaner: remove ruby 1.9.2 code since 1.9.3 is enabled
Merge remote-tracking branch 'origin/bugfix-clean-parameters-from-non-UTF8-chars'