Increase code coverage slightly

An Environment will always have a CompositeDataSource, so there was no way
those exceptions could get raised.

stix2/environment.py

@@ -114,16 +114,10 @@ class Environment(DataStoreMixin):
     create.__doc__ = ObjectFactory.create.__doc__
 
     def add_filters(self, *args, **kwargs):
-        try:
-            return self.source.filters.update(*args, **kwargs)
-        except AttributeError:
-            raise AttributeError('Environment has no data source')
+        return self.source.filters.update(*args, **kwargs)
 
     def add_filter(self, *args, **kwargs):
-        try:
-            return self.source.filters.add(*args, **kwargs)
-        except AttributeError:
-            raise AttributeError('Environment has no data source')
+        return self.source.filters.add(*args, **kwargs)
 
     def parse(self, *args, **kwargs):
         return _parse(*args, **kwargs)

stix2/test/test_properties.py

@@ -16,6 +16,8 @@ def test_property():
     p = Property()
 
     assert p.required is False
+    assert p.clean('foo') == 'foo'
+    assert p.clean(3) == 3
 
 
 def test_basic_clean():