Fix pipeThrough error and track API changes

[aboba]

The current sample throws when attempting to send over a uni-directional stream.

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[jeffposnick]

R: @bashi and @yutakahirano

[bashi]

Thank you for the fix!

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

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[jeffposnick]

Thank you, @aboba & co.! If you could follow the CLA steps outlined in #752 (comment) I'll get this merged.