Fix bbox format in ZOOM_TO_EXTENT reducer

[manisandro]

As far as I can see, various components use/expect the bbox format in the map state to be

map {
 bbox:
  bounds: {
   minx: ...,
   miny: ...,
   maxx: ...,
   maxy: ...
 }
}

The ZOOM_TO_EXTENT reducer however writes a bbox in the format:

map {
 bbox: [minx, miny, maxx, maxy]
}

This can break various things, for instance rotation (since rotation is stored as a field in bbox).

[coveralls]

Coverage decreased (-0.005%) to 80.011% when pulling b945a50 on sourcepole:zoomtoextent into b5b3eb1 on geosolutions-it:master.