string_encoding_compatible_with_regex? method fixed
There was an incorrect argument for Encoding.compatible? - NON_EMPTY_REGEX constant string instead of non_empty_regex regular expression. This caused false encoding incompatibilities and thus extra regular expression encodings.
Also added Regexp::FIXEDENCODING option in non_empty_regex_with_encoding method for preventing "downgrading" regular expression encoding to US-ASCII.
And a little code tweak in tests.