This was actually only a very small change (Levenshtein distance 1). Merged.

Split off from issue371:

In problems without a :metric declaration, we should use 1 for the action
costs rather than 0. (We can keep the info in the begin_metric/end_metric block,
so this wouldn't use any information, just make things clearer for people who
don't look into this block.)

This doesn't violate the current file format.