AuthCodeURL should not include empty values

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use an oauth.Config that doesn't make use of optional fields like Scope, 
AccessType, or ApprovalPrompt.
2. Call config.AuthCodeURL(foo). Useless parameters are included in the URL.

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

I expect to see something like this:

https://example.com/auth?client_id=ClientId&redirect_uri=http%3A%2F%2Flocalhost%
3A8080%2Fhandler&response_type=code&state=foo

But instead I see this:

https://example.com/auth?access_type=&approval_prompt=&client_id=ClientId&redire
ct_uri=http%3A%2F%2Flocalhost%3A8080%2Fhandler&response_type=code&scope=&state=f
oo

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

Please provide any additional information below.

Original issue reported on code.google.com by griffin...@gmail.com on 26 Aug 2014 at 3:40

[GoogleCodeExporter]

https://codereview.appspot.com/130560044

Original comment by a...@golang.org on 26 Aug 2014 at 8:09

• Changed state: Started

[GoogleCodeExporter]

I took a look at your proposed changes and noticed you don't consider scope to 
be an optional parameter, but the client *is* allowed to omit it [1], and that 
seems to me to be the only reasonable behavior for services with no documented 
scope strings. For example, the Harvest API doesn't make any use of it [2].

[1]: http://tools.ietf.org/html/draft-ietf-oauth-v2-31#section-3.3
[2]: https://github.com/harvesthq/api/blob/master/Authentication/OAuth%202.0.md

Original comment by griffin...@gmail.com on 26 Aug 2014 at 3:42

[GoogleCodeExporter]

This issue was closed by revision 80c87b4b6700.

Original comment by a...@golang.org on 27 Aug 2014 at 1:41

• Changed state: Fixed

[GoogleCodeExporter]

Thanks for your feedback. I made the scope and redirect uri fields optional too

Original comment by a...@golang.org on 27 Aug 2014 at 4:08

[GoogleCodeExporter]

Thanks, it was a simple issue but I still didn't expect such a quick turnaround 
time!

Original comment by griffin...@gmail.com on 27 Aug 2014 at 6:15