More Kernel.chop[!] specs.
Encoding related specs for Kernel.chop, .chomp.
Specs for #previous_byte_index.
Cleaned up String#chop[!] specs.
String#chomp, #chop specs for multibyte \n, \r.
Rewrote String#chomp[!] specs.
Fixed String#cho[m]p[!]. Closes #2079.