Fix: class_exists() error when using stdclass with required interfaces

You could quite happily create a mock for a stdClass with required interfaces
however whenever you tried to enforce and requirements you'd be met with

    class_exists() expects parameter 1 to be string, array given

Example:

    $mock = Mockery::mock('stdClass, ArrayAccess');
    $mock->shouldReceive('foo')->once();
    $mock->foo();

Added new test to check for this exact situation, and fix to `Container.php`
which appears to be as simple as checking for `$class` being an array, and in
that instance re-join it based on a comma so that it's a string.

This may or may not be correct as I'm not familiar with Mockery's source, but it
doesn't appear to break any other tests.