Reject X-OC-MTime header if given as a string with hexadecimal notation

In PHP 7.X hexadecimal notation support was removed from "is_numeric",
so "sanitizeMtime" directly rejected those values; in PHP 5.X, on the
other hand, "sanitizeMtime" returned 0 when a string with hexadecimal
notation was given (as it was the behaviour of "intval"). To provide a
consistent behaviour between PHP versions, and given that it does not
make much sense to send X-OC-MTime in hexadecimal notation, now
X-OC-MTime is always rejected if given as a string with hexadecimal
notation.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>