WIP: Add missing type check to method calls

[GuntherRademacher]

The spec, in 4.14.4 Method Calls, requires a map that goes into a method call to have an entry with the given key that is a single function item. Implementation-wise this used to be checked implicitly by the involved dynamic function call, because until recently that required a single function item as well. However with that changed to allowing a sequence of zero or more function items, an extra type check becomes necessary for method calls, which is added here.

This fixes QT4 test cases method-902 and method-904.