Transmission in NETSTANDARD target does not handle partial success.

[cijothomas]

For Netstandard target, transmission does not handle partial response from backend. It simply returns null, so PartialSuccessTransmissionPolicy won't have a chance to resend the rejected items later.
https://github.com/Microsoft/ApplicationInsights-dotnet/blob/develop/src/Microsoft.ApplicationInsights/Channel/Transmission.cs#L171

It also looks like the Transmission class was missing Unit Tests as well as UnitTests was only targeting the full .NET Framework.
https://github.com/Microsoft/ApplicationInsights-dotnet/blob/develop/Test/Microsoft.ApplicationInsights.Test/Shared/Channel/TransmissionTest.cs#L13

Proposal is to remove the multiple implementations, and move everything to use HttpClient and add unit test coverage. (The reason why separate implementation existed earlier was targeting NET40 which is no longer the case)