Changed default CRS for set_extent so it is no longer the geodetic version.

When setting the extent of a plot with ```GeoAxes.set_extent()``` the CRS of the provided coords currently defaults to a geodetic version of ```self.projection``` rather than ```self.projection```. I do not see the reason for this and in my opinion the behaviour is unexpected. I propose changing it so that if I set up an axes in projection X, any provided extent is assumed to be in that coordinate system unless I explicitly tell it otherwise through the  ```crs``` argument.