UtilTest.StringRefCompare: fails if std::char_traits<T>::compare returns a value outside [-1;1]

[jackyf]

Hi,

Thank you for the portability patch merges. Still one new issue on a newest master:

Extract from running tests on Debian s390x and arm64 autobuilders:
-------------------8<--------------------
[ RUN ] UtilTest.StringRefCompare
/«BUILDDIR»/cppformat-2.0.0alpha20151107+ds3/test/util-test.cc:726: Failure
Value of: StringRef("fop").compare(StringRef("foo"))
Actual: 8
Expected: 1
/«BUILDDIR»/cppformat-2.0.0alpha20151107+ds3/test/util-test.cc:727: Failure
Value of: StringRef("foo").compare(StringRef("fop"))
Actual: -8
Expected: -1
[ FAILED ] UtilTest.StringRefCompare (0 ms)
------------------->8---------------------

[vitaut]

Fixed in aa741ba. Thanks!