Response data should always be in a top-level "value" property.

Hi,

Proposal: any response from an end node that contains data should put that
data in a "value" property of a JSON object.

Rationale:

I've been working on improving W3C compliance in the java local end for
selenium, as well as writing a lightweight intermediate server
implementation that streams results where necessary. From this work, it
seems like the current spec is less consistent and requires more processing
work on local ends than the existing JSON wire protocol used by OSS
WebDriver.

Notably, the JSON wire protocol always returns the value from a command in
a top-level property called "value". The W3C spec mostly does this too, except
when it doesn't
<https://github.com/SeleniumHQ/selenium/commit/aa25e5443d29694aa30fd25786a2e85b03262f20#diff-f0a832e4423040d26443b974309b65a3R44>.
This has a number of drawbacks:

1) Processing the result cannot be a two stage process of dumbly extracting
the return value and then coercing to a type. Instead, local ends must know
the command being responded to (information not included in the response)
and switch behaviour as appropriate. This additional complexity is needless.

2) It's inconsistent. Generally, inconsistency is where bugs live.

3) It precludes decorating responses with additional information, since we
cannot know for sure whether the response is one of the ones with a "value"
field, or one where the response body is the data itself.

4) It's a needless divergence from the JSON wire protocol.

The suggested proposal, of always using a "value" field, resolves all of
these issues. The alternative (of never using a "value" field, but instead
always pushing the result data directly into the body of the response)
fails to resolve issues 3 and 4, which seems less than ideal.

Simon