Allow matching against polymorphic collections

[tumbarumba]

As discussed in #252, there are problems with matching against polymorphic collections.

This fix attempts to apply the PECS rule (producer extends, consumer super) to the Hamcrest IsIterableContaining matcher. In this instance, a collection of items should be treated as a producer according to this rule, while a matcher acts as a consumer. There was some confustion about PECS in the context of collections of matchers, but I'm hoping this change addresses those issues.

I'm keen for others to test this work out

[foal]

Nice!
About PECS, INHO it is clear - the itemMatcher is consumer - it consumes the elements of the target collection to check them.It may check the element or any parent of the element (e.g. object != null). The output is producer - it produces global matcher for a whole collection and should be applied to collection of the elements or to collection of element's children.