Fix python3 bug when inserting data-URIs

The current code inserts bytes into a string. In Python 3 those are different types, causing the URI to look like:

    url("data:image/png;charset=utf-8;base64,b'iVBORw0KGgoAAAANSUhEUgAAAA...'")

Because Base64 data is always ASCII it is safe to decode it as such, and insert the resulting string, which causes the correct output:

    url("data:image/png;charset=utf-8;base64,iVBORw0KGgoAAAANSUhEUgAAAA...")

pipeline/compressors/__init__.py

@@ -177,7 +177,7 @@ class Compressor(object):
         if path in self.__class__.asset_contents:
             return self.__class__.asset_contents[path]
         data = self.read_bytes(path)
-        self.__class__.asset_contents[path] = base64.b64encode(data)
+        self.__class__.asset_contents[path] = force_text(base64.b64encode(data))
         return self.__class__.asset_contents[path]
 
     def mime_type(self, path):

tests/tests/test_compressor.py

@@ -46,12 +46,21 @@ class CompressorTest(TestCase):
 
     @patch.object(base64, 'b64encode')
     def test_encoded_content(self, mock):
+        self.compressor.asset_contents.clear()
         self.compressor.encoded_content(_('pipeline/images/arrow.png'))
         self.assertTrue(mock.called)
         mock.reset_mock()
         self.compressor.encoded_content(_('pipeline/images/arrow.png'))
         self.assertFalse(mock.called)
 
+    def test_encoded_content_output(self):
+        self.compressor.asset_contents.clear()
+        encoded = self.compressor.encoded_content(_('pipeline/images/arrow.png'))
+        expected = ('iVBORw0KGgoAAAANSUhEUgAAAAkAAAAGCAYAAAARx7TFAAAAMk'
+                    'lEQVR42oXKwQkAMAxC0Q7rEk5voSEepCHC9/SOpLV3JPULgArV'
+                    'RtDIMEEiQ4NECRNdciCfK3K3wvEAAAAASUVORK5CYII=')
+        self.assertEqual(encoded, expected)
+
     def test_relative_path(self):
         relative_path = self.compressor.relative_path("images/sprite.png", 'css/screen.css')
         self.assertEqual(relative_path, '../images/sprite.png')