Modified MethodTypeHint to transcode short array notations to Traversable

This PR fixes https://github.com/rtens/mockster/issues/9

Instead of ArrayAccess I decided to use Traversable instead since it covers also the usage of Iterator and IteratorAggregate (basically everything which supports foreach-loops).

In my opinion a plain collection does not need to implement ArrayAccess (since it does not support ordered access) but should implement the Iterator interface.