handle unicode characters gracefully in python 2

Author: ananduri9

src/onelogin/saml2/compat.py

@@ -39,6 +39,8 @@ def to_string(data):
 
     def to_bytes(data):
         """ return bytes """
+        if isinstance(data, unicode):
+            return data.encode("utf8")
         return str(data)
 
 else:  # py 3.x