Avoid completing reader or writer except after they are done

[AArnott]

This is a follow-up to #350 which was evidently incompletely fixed. In the earlier fix, we arranged to Complete the reader and writer after they were done, which might be sometime after Dispose is called. This was all done in the base type. In the derived type, a Dispose override method Complete()'d the reader and writer anyway, defeating the very thing we were trying to fix.

The fix is to remove these calls to Complete, leaving the base class to schedule these calls as it already does:

vs-streamjsonrpc/src/StreamJsonRpc/MessageHandlerBase.cs

Lines 64 to 66 in d7135f8

	Task readerDisposal = this.readingCompleted.WaitAsync().ContinueWith((_, s) => ((MessageHandlerBase)s).DisposeReader(), this, CancellationToken.None, TaskContinuationOptions.None, TaskScheduler.Default);
	Task writerDisposal = this.writingCompleted.WaitAsync().ContinueWith((_, s) => ((MessageHandlerBase)s).DisposeWriter(), this, CancellationToken.None, TaskContinuationOptions.None, TaskScheduler.Default);
	this.Completion = Task.WhenAll(readerDisposal, writerDisposal).ContinueWith((_, s) => ((MessageHandlerBase)s).Dispose(true), this, CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);

Fixes #413

[codecov-io]

Codecov Report

Merging #414 into v2.3 will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             v2.3     #414      +/-   ##
==========================================
- Coverage   90.71%   90.67%   -0.04%     
==========================================
  Files          49       49              
  Lines        3716     3711       -5     
==========================================
- Hits         3371     3365       -6     
- Misses        345      346       +1

Impacted Files	Coverage Δ
src/StreamJsonRpc/PipeMessageHandler.cs	90.78% <ø> (-0.57%)	⬇️
src/StreamJsonRpc/MessageHandlerBase.cs	98.73% <0%> (-1.27%)	⬇️

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