Add specs for String#{[],slice} with untrusted Regexp
Add specs for Symbol#{[],slice} with untrusted Regexp
Fix {String,Symbol}#{[],slice} with untrusted Regexp

Symbol#{[],slice} with [Regexp, capture_index] is
depending String#subpattern too.
Merge pull request #2476 from kachick/string,symbol-slicer-infection

{String,Symbol}#{[],slice} with untrusted Regexp should copy the untrusted status to the returning value