opendap driver: use no authentication as default

Apart from historic reasons, using ESGF as the default authentication
method does not seem to be a straight forward choice. Changing the
default to no authentication most likely is the more user friendly
alternative. Apart from that and following intake#57, the default if no
authentication method is given should be anonymous access, but currently
this fall back does not work (see also intake#74).

intake_xarray/opendap.py

@@ -38,7 +38,7 @@ class OpenDapSource(DataSourceMixin):
     """
     name = 'opendap'
 
-    def __init__(self, urlpath, chunks, auth="esgf", xarray_kwargs=None, metadata=None,
+    def __init__(self, urlpath, chunks, auth=None, xarray_kwargs=None, metadata=None,
                  **kwargs):
         self.urlpath = urlpath
         self.chunks = chunks