dont convert extent objects with 'NaN' coordinate values to LatLngBounds

[jgravois]

resolves #879

[patrickarlt]

@jgravois I would actually argue that instead of making latlngbounds in the callback be null, we should make error be something like Invalid Bounds.

This way I can ALWAYS handle latlngbounds as a bounds instead of having to do things like:

task.bounds(function (error, latlngbounds, raw) {
  if (error && latlngbounds) {
    // now I have to check for bounds and an error :(
  }
});

I can do

task.bounds(function (error, latlngbounds, raw) {
  if (error) {
    // request error or invalid bounds....
    return;
  }

  // now I know I have a valid bounds
});

[jgravois]

this is good feedback and incorporated. 🚢 ing it.