remove HttpContextStub and HttpRequestStub from AspNetCore tests (prep FrameworkReference)

[TimothyMothra]

Fix Issue #2251.

I'm preparing to change AspNetCore and AspNetCore.Tests to use FrameworkReference.
There are some incompatibilities in the current tests which I'm resolving here.

Changes

• remove HttpContextStub replace with DefaultContext.
  HttpContextStub formerly inherited DefaultContext, but this class has been marked sealed.
• remove HttpRequestStub.
  HttpRequestStub formerly inherited DefaultHttpRequest, but this class is inaccessible due to it's protection level.

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

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• Design discussion issue #
• Changes in public surface reviewed

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