Fix method signature to allow mocking

[SvenRtbg]

The magic method __call in EpiTwitter deviates from the PHP default:

public function __call($name, $params = null/*, $username, $password*/)

or in short:

public function __call($name, $params = null)

That default value "null" is never been used in PHP. If you call a method without parameters, the $params value is an empty array.

But because of this signature it is impossible to mock that class with e.g. Mockery in strict mode.

It should not change anything if this function is reverted to the standard that PHP sets:

public function __call($name, $params)

For more details on one such problem, see http://stackoverflow.com/questions/21358268/mockery-call-has-a-different-signature