If the layer.srs does not have a srid it should return None

[tlp]

If a srs of a layer does not have a srid it should return None instead of throwing a Type Error.

I am using this dataset: ​http://www.eng.hctx.net/gis/Shapefiles/ZIP CODE.zip

>>> from django.contrib.gis.gdal import DataSource
>>> ds = DataSource("/home/osc/data/zipcode/zipcodep.shp")
>>> layer = ds[0]

In [40]: layer.srs.srid
---------------------------------------------------------------------------
<type 'exceptions.TypeError'>             Traceback (most recent call last)

/home/geo/geo/<ipython console> in <module>()

/home/geo/local/lib/python2.5/site-packages/django/contrib/gis/gdal/srs.py in srid(self)
    206         """
    207         try:
--> 208             return int(self.attr_value('AUTHORITY', 1))
    209         except ValueError:
    210             return None

<type 'exceptions.TypeError'>: int() argument must be a string or a number, not 'NoneType'

[jbronn]

(In [6574]) gis: Fixed #5779, thanks tlp; added type checking to SpatialReference.getitem.