HeaderNames now contains static readonly fields instead of const fields

[halter73]

The static Microsoft.Net.Http.Headers.HeaderNames class contains string fields representing various common header names (e.g. HeaderNames.Origin). Starting in ASP.NET Core 3.0 Preview 5, these will change from const fields to static readonly fields.

While this is not a binary breaking change, source code that used these fields as an argument attribute, 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.

See dotnet/aspnetcore#9514 for discussion.

[This announcement has been migrated to: dotnet/docs#14901]