Simple fixes to mapdist()

[mvkorpel]

These are simple fixes to mapdist(), for example fixing issue #70.

I am aware that a set of more advanced updates is proposed in pull request #129, and this pull request does not play well with that. However, is it desirable to change the semantics of vector origins and destinations so that all pairs of origin-destination (from-to) are searched, as happens in #129? At present, with from=c("A", "B") and to=c("C", "D"), only routes A-C and B-D are searched, but #129 also searches A-D and B-C.

[scottmmjackson]

My vote is to take this over #129 for fixing the issue. I re-ran the issue submitter's example (thank you @avsmith for the reproducible example!) and confirmed this works.