Fix a rare GetAwaiter().GetResult() on a ValueTask in System.Net.Http (dotnet/corefx...

Fix a rare GetAwaiter().GetResult() on a ValueTask in System.Net.Http (dotnet/corefx#41958)

* Fix a rare GetAwaiter().GetResult() on a ValueTask in System.Net.Http

If someone's custom HttpContent-derived type is implementing their async SerializeToStreamAsync with synchronous Write calls on the destination stream, and if that WriteAsync method returns an IValueTaskSource-based ValueTask that's not yet completed, this could throw an exception.   This is unlikely to actually negative impact anyone right now, but it's worth fixing.  This is one of the patterns an analyzer around ValueTasks will help find and fix.

* Update src/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpBaseStream.cs

Commit migrated from https://github.com/dotnet/corefx/commit/f37f05d4a7d916f14980456e9ddb95d2708efa24