Wrong computation of zoom level (when location is bbox and zoom is 'auto')

### Long story short

Briefly, this line https://github.com/dkahle/ggmap/blob/master/R/get_map.R#L178, should read

`zoom <- min(zoomlon, zoomlat)`

instead of

`zoom <- max(zoomlon, zoomlat)`
### Problem

I'm plotting trips on Google Maps as base map. Trips' bboxes are sometimes 'squares' and sometimes 'rectangles'. When they are 'squares' most of the time, zoomlon = zoomlat and the whole trip is plotted.
But when they are 'rectangles', sometimes zoomlon != zoomlat. For the trip to be fully plotted, and not cropped, the min zoom needs to be taken, not the max zoom.

I took that part of code that computes the zoom, changed the line to `min(zoomlon, zoomlat)` and fed the computed zoom to `get_map()` and that solved my issue.

Start in R/get_map.R at line 178, where the automatic zoom is computed for a bounding-box location. Check the behavior with rectangular and square bounding boxes, then verify that the full rectangle remains visible when zoom is set to auto. Done means the zoom calculation no longer crops rectangular trips.

Written by the indexing model from the issue text.