changes toLowercase()/toUppercase() to let more than a-z and 0-9 pass

isLowercase()/isUppercase() failed on whitespace, punctuation marks and non-ascii letters because of a too strict regex check. Now comparing given string to the result of native toLowerCase()/toUpperCase() functions.
Merge pull request #171 from tbasse/fix/isLowerUpperCase

toLowercase() and toUppercase() fail on foreign characters