This was inadvertently removed in https://github.com/mono/mono/commit/611a43ee0f672adbac3e25dc77731843a3d10cf1 from MulticastDelegate.CombineImpl().

Add the type check back in the Delegate base class, we already do the same in Remove().

Add unit tests to cover the behavior to the corlib test suite. (reverted from commit f29878dc713f4e148fe0a1e703fae8929e6fb6e0)

Needed to update some monodoc tests to match that an exception is now thrown again.