fix empty set

for issue https://github.com/GoogleCloudPlatform/gcloud-python/issues/403

for test case. I am curious about whether to create a new test case `test_save_entity_w_transaction_list_value` and included https://github.com/GoogleCloudPlatform/gcloud-python/pull/399.

- Since `Empty` falls in the middle of the sentence it should be lowercase.
- `none` refers to the Python built-in `None`, so should be capitalized
- Change "will be ignore" to "will be ignored".