Default state for unhandledPromptBehavior should be "Dismiss and Notify"

[whimboo]

This issue has been caught by @andreastt while reviewing my patch on https://bugzilla.mozilla.org/show_bug.cgi?id=1264259.

In the patch which enables the prompt behavior support in Firefox I used "Dismiss and Notify" as default state because that is what I thought should be used if the unhandledPromptBehavior capability hasn't been specified for the New Session command.

Then here is what the spec lists when handling such an unexpected alert:

* missing value default state
* not in the table of simple dialogs
      - Dismiss the current user prompt.
      - Return an annotated unexpected alert open error. 

There is the missing value default state used, and no-where is a mention that New Session should set a valid default state. Benefit for that would be that this command could also return the actual mode in the active capabilities, so that clients are informed about the behavior in use.

[andreastt]

I think we should change the default value of the unhandledPromptBehavior capability to dismiss and notify, which is effectively what null evaluates to through the handler’s treatment of the missing value default state today.

Then we should remove the missing value default state and not in the table of simple dialogs branches of handle any user prompts.

This would make the capability behave like other similar capabilities such as pageLoadStrategy, where the matched/negotiated value is returned when the session is created.