oauth2: added shopify endpoint and request validation

[calgoodman]

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[gopherbot]

This PR (HEAD: 111c775) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/213132 to see it.

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[gopherbot]

Message from Gobot Gobot:

Patch Set 1:

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[gopherbot]

Message from Brad Fitzpatrick:

Patch Set 1:

We're not accepting new oauth2 packages:

https://github.com/golang/oauth2/#policy-for-new-packages

Instead, we're adding a new "endpoints" package in:

https://go-review.googlesource.com/c/oauth2/+/212223

So this will need to be updated to use that.

Or feel free to add the endpoints package in this change with just the Shopify variable for now.

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[gopherbot]

Message from Brad Fitzpatrick:

Patch Set 1:

The endpoints package is now submitted. Please move this variable to there. Thanks!

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[gopherbot]

Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

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Please don’t reply on this GitHub thread. Visit golang.org/cl/213132.
After addressing review feedback, remember to publish your drafts!