HeaderNames now contains static readonly fields instead of const fields 

## HeaderNames now contains static readonly fields instead of const fields 

The static [Microsoft.Net.Http.Headers.HeaderNames](https://docs.microsoft.com/en-us/dotnet/api/microsoft.net.http.headers.headernames?view=aspnetcore-2.2) class contains string fields representing various common header names (e.g. [HeaderNames.Origin](https://docs.microsoft.com/en-us/dotnet/api/microsoft.net.http.headers.headernames.origin?view=aspnetcore-2.2)). Starting in ASP.NET Core 3.0 Preview 5, these will change from const fields to static readonly fields.

See aspnet/AspNetCore#9514 for discussion.

### Version introduced

3.0

### Old behavior

These fields used to be `const`

### New behavior

These fields are now `static readonly`.

### Reason for change

The change prevents the values from being embedded across assembly boundaries allowing for value corrections as needed. It also enables faster reference equality checks.

### Recommended action

Recompile against 3.0. Source code that used these fields as an attribute argument, a case in a switch statement, or when defining another constant will no longer be able to do so. To work around this break, developers can switch to using self-defined header name constants or string literals.

### Category

- ASP.NET Core

### Affected APIs

All fields on [Microsoft.Net.Http.Headers.HeaderNames](https://docs.microsoft.com/en-us/dotnet/api/microsoft.net.http.headers.headernames)



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#### Issue metadata

* Issue type: breaking-change

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HeaderNames now contains static readonly fields instead of const fields #14901