Support for type conversions in params array expansion

From this forum post: https://groups.google.com/d/topic/exceldna/l-grehDBLng/discussion.
Currently we only support params arrays of primitive types - the expansion does not integrate with other type conversions. In preference to the suggested implementation in the forum post, I'd rather ensure that an array conversion MyType[] to object[] can be done before the params expansion. The params expansion needs to leave the types as 'object' in order for the 'missing' values to be recognized.

It needs a bit of thought still...