Force integer value in asserters\adapter\call::exactly()

This fixes : https://github.com/atoum/atoum/pull/320#discussion_r10975338

I chose to do a cast on the `$number` argument of the `asserters\adapter\call::exactly()` method to keep the original behavior.

Previously, no checks were done on the argument, so passing a float would have made the assertion fail. Now we cast the value to `int` and then check the actual value against the casted argument.