[Azure Search] Prevent ObjectDisposedException when Indexing using "Merge" operation

[arv100kri]

In the DoIndexAsync method, when the response contains partial success/failure - an IndexBatchException object is constructed, whose request and response wrapper objects are assigned to the result from the underlying proxy's operation.

The request wrapper object attempts to re-read the request content from the underlying proxy's result - this results in the ObjectDisposedException when the target framework is .NET Framework 4.x

This is because the HttpClient in the .NET framework, automatically disposes the HttpContent once the request is actually sent, whereas the HttpClient in .NET Core no longer disposes the HttpContent

This pull request adds conditional logic to detect the target framework, and re-serializes the request payload in case the framework is .NET Framework 4.x

Fixes #6910

FYI: @weshaggard