This changes makes encoder.Close aware of how many bytes to write if there
is any data left in the buffer.

Fixes #25295

Change-Id: I4138891359935509cb561c453b8059ba2b9e576b
GitHub-Last-Rev: f374096d2f3cae8635506074f59e1cd440c14844
GitHub-Pull-Request: golang/go#25316
Reviewed-on: https://go-review.googlesource.com/112515
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>