Fix build_full_result for clients

[kyleknap]

Fixes build_full_result() for clients.

Before this would happen:

>>> import botocore.session
>>> session = botocore.session.get_session()
>>> client = session.create_client('s3')
>>> paginator = client.get_paginator('list_objects')
>>> pages = paginator.paginate(Bucket='mybucketfoo')
>>> pages.build_full_result()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "botocore/paginate.py", line 215, in build_full_result
    for _, page in self:
ValueError: too many values to unpack

Now this lists all of my objects.

Note that I only included that one unit test for clients to have a smoke test even though operation objects have much more tests. I was considering switching all of the operation object pagination test to clients, but I was hesitant to do that since the CLI still uses operation object pagination. Let me know what you think.

cc @jamesls @danielgtaylor

[jamesls]

[coveralls]

Coverage increased (+0.0%) to 94.55% when pulling 7867b40 on kyleknap:build-full-result-clients into d2792df on boto:develop.