Allow to mock the same function with defferent arguments in the namespace

[michalbundyra]

In prophecy we can mock the same class function with different arguments in case it's called multiple times, for example:

$prophecy = $this->prophesize(DateTime::class);
$prophecy->format('Y-m-d')->willReturn('2018-03-19');
$prophecy->format('H:i:s')->willReturn('12:00:00');
$obj = $prophecy->reveal();

$this->assertSame('2018-03-19', $obj->format('Y-m-d'));
$this->assertSame('12:00:00', $obj->format('H:i:s'));

Now, with that library is not possible to mock the same function, because we get error:
phpmock\MockEnabledException: FUNCTION_NAME is already enabled. Call disable() on the existing mock.

This PR resolve that issue.

ping @malkusch

[malkusch]

Someone else has to do that. See #7