- use `IDictionary<string, TValue>` support in `<a/>` and `<form/>` tag helpers
- make `RouteValues` dictionaries `IDictionary<string, string>` for ease of use
- remove `TagHelperOutputExtensions.FindPrefixedAttributes()`
- set all `GeneratedTagHelperContext` properties
- add error for tag helper dictionary properties where `TValue` is `ModelExpression`
- add new `RazorPage.InvalidTagHelperIndexerAssignment()` method and resource
tests
- use new `isIndexer` argument when creating `TagHelperAttributeDescriptor`
- arrange `AnchorTagHelper` and `FormTagHelper` correctly
- also expect `routeValues != null` in calls to `IHtmlGenerator`
nits:
- get rid of some `foo` and `bar` gunk in tests
- remove unused variable to cleanup a test compilation warning
