printable filter should replace non-ascii bytes

In Python 2, when repr() returns bytes, replace any non-ascii bytes
with the unicode ? character to ensure that the result is printable.

Fixes #66

flask_debugtoolbar/__init__.py

@@ -30,7 +30,10 @@ def replace_insensitive(string, target, replacement):
 
 def _printable(value):
     try:
-        return repr(value)
+        value = repr(value)
+        if isinstance(value, bytes):
+            value = value.decode('ascii', 'replace')
+        return value
     except Exception as e:
         return '<repr(%s) raised %s: %s>' % (
                object.__repr__(value), type(e).__name__, e)