[server][FaceRest] Replace strcasecmp() with StringUtils::casecmp() to be simple.

The specification of C library's strcasemp() is complicated.
In addition, StringUtils::casecmp() can accept a string type variable
directly.
Merge pull request #96 from kz0817/issue58-2

[server][FaceRest] Use StringUtils::casecmp() to be simple (omit '== 0').