utils: remove use of assert

When code is optimized, the assertion will be removed.

Change-Id: I3d313ef637646ca10b721acd3bf91795ed2a3293

ooi/utils.py

@@ -22,14 +22,17 @@ import six.moves.urllib.parse as urlparse
 def utf8(value):
     """Try to turn a string into utf-8 if possible.
 
-    Code is modified from the utf8 function in
+    The original code was copied from the utf8 function in
     http://github.com/facebook/tornado/blob/master/tornado/escape.py
 
     """
-    if isinstance(value, six.text_type):
-        return value.encode('utf-8')
-    assert isinstance(value, str)
-    return value
+    if value is None or isinstance(value, six.binary_type):
+        return value
+
+    if not isinstance(value, six.text_type):
+        value = six.text_type(value)
+
+    return value.encode('utf-8')
 
 
 def join_url(base, parts):