PropertyFunctionGenericListReturn fails in xplat

[rainersigwald]

This test is expected to return the string "A;B;C;D" but actually returns "System.Collections.Generic.List1[System.String]". Clearly we've changed something to cause aListto not get flattened, but it's not immediately obvious where. I was suspicious of changes from 1da7bfd toExpander`, but since the test fails on both full-framework and Core, that doesn't appear to be the problem.

[rainersigwald]

After further investigation, return types of property functions in general seem to be broken. Many tests in Expander_Tests are failing for this reason.