In wfArrayDiff2_cmp only compare $a and $b as string if they both are strings

Noticed during bug 54662

Warning: strcmp() expects parameter 2 to be string, array given in
/usr/local/apache/common-local/php-1.22wmf19/includes/GlobalFunctions.php on
line 127

Bug: 54662
Change-Id: Icdd7fa229e49d7e83aa2a7992895f497806448e1
Merge "In wfArrayDiff2_cmp only compare $a and $b as string if they both are strings"