Entity should not require a Key

[dhermes]

Relates to
https://docs.google.com/document/d/1xKw8Tz6lfN5uOcv0my4uUgFSeDoRzwCLho2UGlBpqDU/edit

From the spec:

An entity must have a key, unless otherwise documented (for example, an entity in
<code>Value.entityValue</code> may have no key). An entity's kind is its key's path's
last element's kind, or null if it has no key.

Though not required, we should be opinionated (in the constructor) about when it's not necessary if possible.

[tseaver]

I'm not sure we should enable direct creation of an Entity w/o a key: such objects wouldn't be able to satisfy the rest of the contract (save(), etc.). When marshalling a "sub-entity" from value.entityValue, wouldn't it be more natural just to have that property's value be a dict?

[dhermes]

I just filed this based on @Alfus comments. Any thoughts @Alfus?

[Alfus]

In an entity value, the key can be present, partial or missing. This lets the value handle dicts (no key) and entities (with partial or complete keys).

[dhermes]

This is not an issue any longer, the Key is not required and failure occurs naturally in methods than need a Key.