Improve test coverage of SocketsHttpHandler (dotnet/corefx#27135)

Improve test coverage of SocketsHttpHandler (dotnet/corefx#27135)

* Improve test coverage of SocketsHttpHandler

Used code coverage information to write targeted tests to cover various uncovered paths in SocketsHttpHandler.  Along the way fixed a variety of issues relating to throwing the wrong exception type, a stack overflow due to an unexpected recursive call, etc.

* Address PR feedback

* Re-disable a test

Commit migrated from https://github.com/dotnet/corefx/commit/39677219aaeab8c6591b625ab2874acb17eaf4ef