Improve code coverage slightly

Environment will always have a CompositeDataSource, so the try/catches
in add_filter/s did not make sense.

stix2/environment.py

@@ -156,16 +156,10 @@ class Environment(DataStoreMixin):
     set_default_object_marking_refs.__doc__ = ObjectFactory.set_default_object_marking_refs.__doc__
 
     def add_filters(self, *args, **kwargs):
-        try:
         return self.source.filters.update(*args, **kwargs)
-        except AttributeError:
-            raise AttributeError('Environment has no data source')
 
     def add_filter(self, *args, **kwargs):
-        try:
         return self.source.filters.add(*args, **kwargs)
-        except AttributeError:
-            raise AttributeError('Environment has no data source')
 
     def parse(self, *args, **kwargs):
         return _parse(*args, **kwargs)

stix2/test/test_workbench.py

@@ -193,6 +193,10 @@ def test_workbench_related_with_filters():
     assert resp[0].name == malware.name
     assert resp[0].created_by_ref == IDENTITY_ID
 
+    # filters arg can also be single filter
+    resp = get(MALWARE_ID).related(filters=filters[0])
+    assert len(resp) == 1
+
 
 def test_add_data_source():
     fs_path = os.path.join(os.path.dirname(os.path.realpath(__file__)), "stix2_data")