use vcr to mock tests

[mpadge]

The problems highlighted in osmdata issues #126 and #128 are pervasive, and affect CRAN systems too, causing tests to fail. vcr is finally on CRAN, so can be used to mock tests far more neatly and comprehensively than current approach.

[maelle]

Note that @sckott has started working on a whole book about HTTP testing in packages! https://github.com/ropensci/http-testing-book