- aspnet/Mvc#3138 part 2/2
- request's Content-Type header must be a subset of what an `IInputFormatter` can consume
- `[Consumes]` is similar
- what an `IOutputFormatter` produces must be a subset of the request's Accept header
- `FormatFilter` and `ObjectResult` are similar
- `ObjectResult` no longer falls back to `Content-Type` header if no `Accept` value is acceptable
- left `WebApiCompatShim` code alone for consistency with down-level `System.Net.Http.Formatting`
- correct tests to match new behaviour
- do not test `Accept` values containing a `charset` parameter; that case is not valid
WIP:
- four test failures; something about comparing media types w/ charset included
- why do some localization tests fail in VS?
nits:
- add `InputFormatterTests`
- add / update comments and doc comments
- correct xUnit attributes in `ActionResultTest`; odd it doesn't show up in command-line runs
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