gcloud.connection:  57% -> 100% coverage.
gcloud.credentials:  0% -> 100% coverage.
Refactor test jigs for reuse.

Also, back out the class-level knob for testing;  prefer instead a
controlled monkey-patch of the module.
gcloud.datastore:  40% -> 100% coverage.
Normalize w/ gcloud.connection.Connection.
gcloud.datastore.connection:  27% -> 52% coverage.
gcloud.datastore.connection:  52% -> 61% coverage.
gcloud.datastore.connection:  61% -> 72% coverage.
Split out large-dict assertions.
gcloud.datastore.connection:  72% -> 78% coverage.
gcloud.datastore.connection:  78% -> 93% coverage.
gcloud.datastore.connection:  93% -> 100% coverage.
gcloud.datastore.dataset:  60% -> 70% coverage.
gcloud.datastore.dataset:  70% -> 100% coverage.
Clarify coverage for gcloud.datastore.entity.
gcloud.datastore.entity:  67% -> 73% coverage.

Fix bug in 'Entity.reload()' found by improving coverage. ;)
gcloud.datastore.entity:  73% -> 75% coverage.
gcloud.datastore.entity:  75% -> 94% coverage.
gcloud.datastore.entity:  94% -> 100% coverage.
gcloud.datastore.helpers:  81% -> 100% coverage.