Fix odata batch support

[cfranchuk]

David, I re-implemented many of the same changes as before to fix the OData Batch support for .NET Core and .NET Standard.

This time I went with the approach of implementing the least changes to existing projects to maintain backwards compatibility as much as possible.

Full Framework

• Nuget packages and dependencies are NOT changed.
• Batch logic was modified to explicitly set the HTTP header version to 1.1 because the newer 'HttpRequestMessage' will default the HTTP header version to 2.0 which will cause an error.

.NET Core

• Upgraded the framework to 2.1 LTS.
• Upgrade Newtonsoft from 9.x to 11.x

.NET Standard

• Dropped support for .NET standard 1.4.
• Existing Nuget package versions remain the same.

Added a .NET core unit test project to verify that equivalent functionality exists between .NET Full Framework and .NET Core.