Fix #168: Subsegment name serialization causing MissingName error

Django sometimes uses SafeString instead of str for template names.

Jsonpickle serializes SafeString as `{ "py/newargs": ["django/forms/widgets/attrs.html"]}`
instead of `"django/forms/widgets/attrs.html" which is the reason for
the invalid subsegment name.

Author: Kevin Michel

aws_xray_sdk/ext/django/templates.py

@@ -1,6 +1,7 @@
 import logging
 
 from django.template import Template
+from django.utils.safestring import SafeString
 
 from aws_xray_sdk.core import xray_recorder
 
@@ -22,6 +23,10 @@ def xray_render(self, context):
         template_name = self.name or getattr(context, 'template_name', None)
         if template_name:
             name = str(template_name)
+            # SafeString are not properly serialized by jsonpickle,
+            # turn them back to str by adding a non-safe str.
+            if isinstance(name, SafeString):
+                name += ''
             subsegment = xray_recorder.current_subsegment()
             if subsegment:
                 subsegment.name = name