[whatwg/streams] Do not assert() when writer is detached during close() (#621)

When writer.releaseLock() was called during us.close(), an assert would
fire in WritableStreamFinishClose() as it expected the writer to still
be attached.

Since detaching the writer during close() is permitted, remove the
assert.

Also add tests that this works correctly.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/621

-- Commit Summary --

  * Verify that releaseLock() during close() works
  * Do not assert() when writer is detached during close()

-- File Changes --

    M reference-implementation/lib/writable-stream.js (4)
    M reference-implementation/to-upstream-wpts/writable-streams/close.js (32)

-- Patch Links --

https://github.com/whatwg/streams/pull/621.patch
https://github.com/whatwg/streams/pull/621.diff

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