Updated the BOOLFromNSString

In case a string is returned, it is also very likely to include YES, Yes, yes, true, True or True as possible values.
Check string for nil value

make sure to check the string for nil value before doing the caseInsensitiveCompare, otherwise this could introduce a bug.
Merge pull request #209 from NSMutableString/patch-1

Updated the BOOLFromNSString