x/oauth2: fix Wunderlist api usage (easy fix)

[bubuntux]

golang/oauth2#172
there is the fix, one line change
Wunderlist api needs to send the client_secret when requesting the token, therefore this change is need it to work with it (https://developer.wunderlist.com/documentation/concepts/authorization)

[dmitshur]

FYI, the patch you've sent in golang/oauth2#172 is missing a trailing comma after the string.

[ianlancetaylor]

CC @adg

[gopherbot]

CL https://golang.org/cl/19784 mentions this issue.