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 e4bd8e3a17d16cde919d5afdfc0fe2878f554e5c on badlydrawnrod:master into c27084114e258863b82753fc574a362cd6c62fcd on mattupstate:master.

[coveralls]

Coverage remained the same at 95.652% when pulling ca7506cba6c397b3685cfaed175c83a8d7bb6193 on badlydrawnrod:master into c27084114e258863b82753fc574a362cd6c62fcd on mattupstate:master.

[coveralls]

Coverage remained the same at 95.652% when pulling 021cf94966f601f5bd65d61cfa06894a25895a91 on badlydrawnrod:master into c27084114e258863b82753fc574a362cd6c62fcd on mattupstate:master.

[stephane]

Already fixed by #63 and many others.