- #2793
- add `ICollectionModelBinder`, allowing `GenericModelBinder` to call `CreateEmptyCollection()`
- adjust `CollectionModelBinder` and `DictionaryModelBinder` to activate model if default types are incompatible
- do not create default (empty) top-level collection in fallback case if Model already non-`null`
- change type checks in `GenericModelBinder` to align with `CollectionModelBinder` capabilities
- add special case for `IEnumerable<T>`
- correct `ModelMetadata` of a few tests that previously did not need that information
