convert link.href to string

When I used this code with Google Appengine it didn't work because the redirect_url was 'unicode'. I propose a really simple change : convert the redirect_url to a string
Merge branch 'patch-1' of github.com:mikefromlyon69/rest-api-sdk-python into mikefromlyon69-patch-1

Google app engine unicode issue
Document app engine str unicode issue
Merge branch 'mikefromlyon69-patch-1'

merge in google appengine fix