- #1418
- add new fallback binding in `DictionaryModelBinder`
- similar to MVC 5 approach but more explicit and with better key conversion support
- fix bugs in `PrefixContainer` encountered while adding new tests of #1418 scenarios
- did not handle entries like "[key]" or "prefix.key[index]" correctly
- refactor part of `GetKeyFromEmptyPrefix()` into `IndexOfDelimiter()`; share with `GetKeyFromNonEmptyPrefix()`
- extend `ReadableStringCollectionValueProviderTest` to cover bracketed key segments
nits:
- remove use of "foo", "bar", and "baz" in affected test classes
- `""` -> `string.Empty`
- `vpResult` -> `result`
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