Fix for a small bug that causes scopes to be prefixed by a bunch
of `+` chars, which results in an invalid scopes response from
the Jira Auth Server.
Change-Id: I00ded66c1eeb042a02f11ef3c84d7281159b1039
GitHub-Last-Rev: f76f48058979f4c7690b702a4fb31f245f9f7043
GitHub-Pull-Request: golang/oauth2#282
Reviewed-on: https://go-review.googlesource.com/110917
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>