fix default proof type selection

[chunningham]

Since #253 we have had good automatic defaults for verificationMethod and verificationRelationship, however auto-defaults didn't work for type as long as the signature suite was chosen prior to ensure_or_pick_verification_relationship, as that fn modifies options.type_ to be appropriate.

[cobward]

... ensure_or_pick_verification_relationship, as that fn modifies options.type_ to be appropriate.

I assume this is a typo, but just to make sure: that fn doesn't modify options.type_, but it can modify options.verification_method. Is this sufficient to improve the default proof type selection?

[sbihel]

I'm not really knowledgeable about that part but it seems reasonable