- #1758
- provide the property bag in `ModelMetadata`; seal it in `CachedModelMetadata`
- add unit tests
- include use of `AdditionalValues` in model binding functional test
nits:
- expose `AdditionalValues` as an `IDictionary` though MVC 5 uses `Dictionary`
- seal `ModelMetadata.Properties` collection as well
- cover a few properties previously missed in `CachedDataAnnotationsModelMetadataTest`
- correct two `ModelMetadataTest` method names
Doug Bunting