Fix a macher to check "not subclass's instance" in String#+
Fix a macher to check "not subclass's instance" in String#%
Fix a macher to check "not subclass's instance" in String#crypt
Merge pull request #229 from kachick/fix-critical_misused_matchers_in_string

Use `be_an_instance_of` instead of `be_kind_of` for checking "not subclass's instance" in String