Filter::Regexp now works on each entry as intended

Encode::decode overwrites the source string when FB_CROAK is used as the
CHECK flag.  This sets the 'regex' config value to an empty string after
processing the first entry, which means the regex never runs on
subsequent entries.  Adding LEAVE_SRC to the CHECK flag ensures the
plugin's config data isn't touched.
Merge pull request #8 from tsibley/filter-regexp-multiple-entries

Filter::Regexp now works on each entry as intended