Editorial: Correct "process capabilities" algorithm

[jugglinmike]

Correct the derivation of "required capabilities" and simplify the return value.

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[jugglinmike]

As written today (that is, before applying this patch), "process capabilities" and "match capabilities" are incoherent. "process capabilities" may invoke "match capabilities" with a value of null, to which the latter attempts to apply the "iterate" algorithm for ordered maps.

This occurs if either of the following conditions is met:

• the "session.new" command is called with a "params" object which lacks a property named" "capabilities"
• the "session.new" command is called with a "params" object whose "capabilities" property is an object which lacks a property named "alwaysMatch"

Since "process capabilities" was modeled after an algorithm of the same name in the WebDriver specification, and since I'm not aware of any reason to diverge, I designed this patch to resolve the incoherency by mimicking the behavior in WebDriver. That is: when the "session.new" command is invoked without any specified capabilities, then the session is created successfully.

With this patch applied, "process capabilities" always invokes "match capabilities" with a map, though the map may be empty. "match capabilities" continues to tolerate empty maps.

[jugglinmike]

Thanks, @jkva!