Provide no default value of run_arcseconds.

The correct behaviour when the radius of the sun in arcseconds can
not be found is to not know what it is, not to assume that the observer
is at the average sun-earth distance.

sunpy/map/mapbase.py

@@ -245,10 +245,10 @@ def yrange(self):
 
     @property
     def center(self):
-        """Returns the offset between the center of the Sun and the center of 
+        """Returns the offset between the center of the Sun and the center of
         the map."""
-        return {'x': wcs.get_center(self.shape[1], self.scale['x'], 
-                                    self.reference_pixel['x'], 
+        return {'x': wcs.get_center(self.shape[1], self.scale['x'],
+                                    self.reference_pixel['x'],
                                     self.reference_coordinate['x']),
                 'y': wcs.get_center(self.shape[0], self.scale['y'],
                                     self.reference_pixel['y'],
@@ -263,7 +263,7 @@ def rsun_meters(self):
     def rsun_arcseconds(self):
         """Radius of the sun in arcseconds"""
         return self.meta.get('rsun_obs', self.meta.get('solar_r',
-                                         self.meta.get('radius', constants.average_angular_size.to('arcsec').value)))
+                                         self.meta.get('radius', None)))
 
     @property
     def coordinate_system(self):
@@ -481,7 +481,7 @@ def resample(self, dimensions, method='linear'):
         new_map.data = new_data
         new_map.meta = new_meta
         return new_map
-    
+
     def rotate(self, angle=None, rmatrix=None, scale=1.0,
                rotation_center=(0, 0), recenter=False,
                missing=0.0, interpolation='bicubic', interp_param=-0.5):