oauth2 Add ability to use non-escaped header auth style

Some OAuth providers do not adhere to the standard of accepting query escaped credentials when using Basic header auth. If your client ID or client secret contains non-standard URL query characters they will be rejected by the OAuth provider. The current implementation always query escapes the client_id and client_secret: req.SetBasicAuth(url.QueryEscape(clientID), url.QueryEscape(clientSecret)) This PR proposes a new AuthStyle AuthStyleInHeaderNoEscape that will pass the credentials unescaped. This has been raised several times: https://github.com/golang/oauth2/issues/251 https://github.com/golang/oauth2/issues/318 https://github.com/golang/oauth2/issues/320 but without this feature this library cannot be used against some key OAuth providers.

This PR replaces https://github.com/golang/oauth2/pull/351 which was raised in an early version of the code.

Feb 12 '21 16:02 mwielbut

This PR (HEAD: 04dcc31dfd44a128d8d6759c51c3c22038fb09c9) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/291529 to see it.

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Feb 12 '21 16:02 gopherbot

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