Update StringUtils and StringUtilsTest.

Added methods strip(), isSecure() and isVaildIsbn13() in StringUtils.
Added testcases in StringUtilsTest.
Testcases for isBlank() created.
Testcases for join() created.

Methode join() implementiert
Testcases for isValidISBN10() added.

Methode isValidISBN10() impementiert
Removed unnessecery method isEmpty() -> isBlank() is the method to use.
Testcommit to check ssh-key. No change of source.