Making Blob.exists() work as Bucket.exists().

[dhermes]

Also returning the _BucketIterator directly in
storage.api.list_buckets() rather than wrapping it in iter().

@tseaver As mentioned #812 (comment) I said I'd fix Blob.exists() while making sure the iterators didn't work in batches.

It turns out they were already not working. Here is a working get request

>>>fromgcloudimport_helpers>>>fromgcloudimportstorage>>>>>>_helpers._PROJECT_ENV_VAR_NAME='GCLOUD_TESTS_PROJECT_ID'>>>storage.set_defaults()
>>>>>>withstorage.Batch() asbatch:
... bucket=storage.get_bucket('bucket-name')
... printbucket._properties
... <gcloud.storage.batch._FutureDictobjectat0x7f7ce19f4a10>>>>bucket._properties
{u'kind': u'storage#bucket', u'name': u'bucket-name', ...

while a bucket iterator fails since _FutureDict.get throws a KeyError:

>>>withstorage.Batch() asbatch:
... buckets_iter=storage.list_buckets()
... response=buckets_iter.get_next_page_response()
... Traceback (mostrecentcalllast):
File"<stdin>", line3, in<module>File"gcloud/storage/iterator.py", line115, inget_next_page_responseself.next_page_token=response.get('nextPageToken')
File"gcloud/storage/batch.py", line96, ingetkey, default))
KeyError: "Cannot get('nextPageToken', default=None) on a future"

and a blob iterator fails (but not quite always)

>>>bucket.connection<gcloud.storage.batch.Batchobjectat0x7f3b1c56db50>>>>bucket.connection._connection<gcloud.storage.connection.Connectionobjectat0x7f3b1c56d490>>>>bucket._connection=None>>>>>>withstorage.Batch() asbatch:
... blobs_iter=bucket.list_blobs()
... response=blobs_iter.get_next_page_response()
...
Traceback (mostrecentcalllast):
File"<stdin>", line3, in<module>File"gcloud/storage/iterator.py", line111, inget_next_page_responseresponse=self.connection.api_request(
AttributeError: 'NoneType'objecthasnoattribute'api_request'

[coveralls]

Coverage remained the same at 100.0% when pulling bf8b655 on dhermes:fix-blob-exists into aafcd2a on GoogleCloudPlatform:master.

[tseaver]

LGTM