goauth2: add soundcloud to the list of non-compliant oauth2 providers

[GoogleCodeExporter]

What steps will reproduce the problem?

try oauth with soundcloud api (https://api.soundcloud.com), after user accepts,
exchange provided code for soundcloud access_token with Transport.Exchange()

What is the expected output? What do you see instead?

Expected: getting the access token from Soundcoud
Getting Error 401 invalid client id

What version of the product are you using? On what operating system?

afe77d958c70 on OSX Yosemite

Please provide any additional information below.

Same issue as #39 for instagram, solution is to add soundcloud's base API URL 
to providerAuthHeaderWorks. See patch attached.

Original issue reported on code.google.com by [email protected] on 24 Feb 2015 at 8:39

Attachments:

• patch

[GoogleCodeExporter]

Nevermind that, just saw that it is now on golang.org/x/ and the soundcloud fix 
is in there.

Original comment by [email protected] on 24 Feb 2015 at 9:37