all: deprecate NoContext

There is no good reason why we suggest NoContext rather than
context.Background(). When the oauth2 library first came around, the
community was not familiar with the x/net/context package. For
documentation reasons, we decided to add NoContext to the oauth2
package. It was not a good idea even back then. And given that context
package is fairly popular, there is no good reason why we are
depending on this.

Updating all the references of NoContext with context.Background
and documenting it as deprecated.

Change-Id: I18e390f1351023a29b567777a3f963dd550cf657
Reviewed-on: https://go-review.googlesource.com/27690
Reviewed-by: Chris Broadfoot <cbro@golang.org>

oauth2.go

@@ -21,6 +21,8 @@ import (
 
 // NoContext is the default context you should supply if not using
 // your own context.Context (see https://golang.org/x/net/context).
+//
+// Deprecated: Use context.Background() or context.TODO() instead.
 var NoContext = context.TODO()
 
 // RegisterBrokenAuthHeaderProvider registers an OAuth2 server