[BUG FIX] [MINOR] Fix encoding with unicode characters.

When passing a unicode string to builder.CreateString, the
default encoding assumed all characters can be encoded
using ascii. Added a fix so a user can specify the
encoding and how to handle errors when creating strings.

python/flatbuffers/builder.py

@@ -361,14 +361,14 @@ class Builder(object):
         self.PlaceUOffsetT(vectorNumElems)
         return self.Offset()
 
-    def CreateString(self, s):
+    def CreateString(self, s, encoding='utf-8', errors='strict'):
         """CreateString writes a null-terminated byte string as a vector."""
 
         self.assertNotNested()
         self.nested = True
 
         if isinstance(s, compat.string_types):
-            x = s.encode()
+            x = s.encode(encoding, errors)
         elif isinstance(s, compat.binary_type):
             x = s
         else: