Fix bug related to `text` field

Previously under python 2.x the text field allowed through values which
would not be turned into a string. This is invalid as all output should
be a valid textual string. This commit fixes that behavior to properly
cast the value to a string in all cases.

disco/types/base.py

@@ -224,9 +224,7 @@ def text(obj):
     if six.PY2:
         if isinstance(obj, str):
             return obj.decode('utf-8')
-        return obj
+    return six.text_type(obj)
-    else:
-        return str(obj)
 
 
 def with_equality(field):

tests/types/base.py

@@ -1,6 +1,6 @@
 import pytest
 
-from disco.types.base import Model, Field, cached_property
+from disco.types.base import Model, Field, cached_property, text
 
 
 @pytest.fixture
@@ -39,3 +39,14 @@ def test_defaults():
     model = TestModel()
     assert model.a is None
     assert model.b == 0
+
+
+def test_text_casting():
+    class TestModel(Model):
+        a = Field(text)
+
+    model = TestModel({'a': 1})
+    assert model.a == '1'
+
+    model = TestModel({'a': {}})
+    assert model.a == '{}'