clientcredentials: allow override of grant_type

[twpayne]

Password-based authentication to the Keycloak API requires grant_type to be password. It would be very helpful if golang.org/x/oauth2 could be used for this, and all's that missing is the ability to override grant_type.

Fixes #283

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Patch Set 1:

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Message from Brad Fitzpatrick:

Patch Set 1:

Please add a test in the existing test file.

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Message from Tom Payne:

Patch Set 2:

Thanks for the feedback. Test added. Are any other changes needed?

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Message from Brad Fitzpatrick:

Patch Set 3:

(1 comment)

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This PR (HEAD: 0e6f85e) has been imported to Gerrit for code review.

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Message from Tom Payne:

Patch Set 4:

Test added, as requested.

Possibly a new record for test/code ratio: 38 lines of test for 21 characters of change :)

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Message from Brad Fitzpatrick:

Patch Set 4: Run-TryBot+1

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Patch Set 4:

TryBots beginning. Status page: https://farmer.golang.org/try?commit=65d58757

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Patch Set 4: TryBot-Result+1

TryBots are happy.

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This PR is being closed because golang.org/cl/158517 has been merged.