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)

test/test_toolbar.py

@@ -1,3 +1,10 @@
+import sys
+
+import pytest
+
+from flask_debugtoolbar import _printable
+
+
 def load_app(name):
     app = __import__(name).app
     app.config['TESTING'] = True
@@ -9,3 +16,26 @@ def test_basic_app():
     index = app.get('/')
     assert index.status_code == 200
     assert b'<div id="flDebug"' in index.data
+
+
+@pytest.mark.skipif(sys.version_info >= (3,),
+                    reason='test only applies to Python 2')
+def test_printable_unicode():
+    class UnicodeRepr(object):
+        def __repr__(self):
+            return u'\uffff'
+
+    printable = _printable(UnicodeRepr())
+    assert "raised UnicodeEncodeError: 'ascii' codec" in printable
+
+
+@pytest.mark.skipif(sys.version_info >= (3,),
+                    reason='test only applies to Python 2')
+def test_printable_non_ascii():
+    class NonAsciiRepr(object):
+        def __repr__(self):
+            return 'a\xffb'
+
+    printable = u'%s' % _printable(NonAsciiRepr())
+    # should replace \xff with the unicode ? character
+    assert printable == u'a\ufffdb'