Fix attribute error in default payload handler.

[badlydrawnrod]

If the authentication handler returned a dictionary rather than an
object then the default payload handler would throw an AttributeError
as no default value was supplied for getattr().

[coveralls]

Coverage remained the same at 95.652% when pulling e4bd8e3 on badlydrawnrod:master into c270841 on mattupstate:master.

[coveralls]

Coverage remained the same at 95.652% when pulling ca7506c on badlydrawnrod:master into c270841 on mattupstate:master.

[coveralls]

Coverage remained the same at 95.652% when pulling 021cf94 on badlydrawnrod:master into c270841 on mattupstate:master.

[stephane]

Already fixed by #63 and many others.