Vector.rasterize() raises unwanted UserWarning

[rhugonnet]

When using out_value=np.nan (which is fairly common practice, to have only rasterized values as valid), rasterize raises:

UserWarning: Setting default nodata -99999 to mask non-finite values found in the array, as no nodata value was defined.
  warnings.warn(

We should either silence this warning by default, probably adding an argument to give the user the option to define the nodata of the new raster.