BOM-617 Fix test_static_replace test failures.

Update the tests around static_file code to use byte streams instead of
string streams for generating static content.  This is a fix to get
these tests passing on python 3.

common/djangoapps/static_replace/test/test_static_replace.py

@@ -3,7 +3,7 @@
 from __future__ import absolute_import, print_function
 
 import re
-from six import StringIO
+from six import BytesIO
 from six.moves.urllib.parse import parse_qsl, urlparse, urlunparse
 
 import ddt
@@ -331,7 +331,7 @@ class CanonicalContentTest(SharedModuleStoreTestCase):
             StaticContent: the StaticContent object for the created image
         """
         new_image = Image.new('RGB', dimensions, color)
-        new_buf = StringIO()
+        new_buf = BytesIO()
         new_image.save(new_buf, format='png')
         new_buf.seek(0)
         new_name = name.format(prefix)
@@ -355,7 +355,7 @@ class CanonicalContentTest(SharedModuleStoreTestCase):
             StaticContent: the StaticContent object for the created content
 
         """
-        new_buf = StringIO('testingggggggggggg')
+        new_buf = BytesIO(b'testingggggggggggg')
         new_name = name.format(prefix)
         new_key = StaticContent.compute_location(cls.courses[prefix].id, new_name)
         new_content = StaticContent(new_key, new_name, 'application/octet-stream', new_buf.getvalue(), locked=locked)

common/lib/xmodule/xmodule/contentstore/content.py

@@ -270,7 +270,7 @@ class StaticContent(object):
             if query_val.startswith("/static/"):
                 new_val = StaticContent.get_canonicalized_asset_path(
                     course_key, query_val, base_url, excluded_exts, encode=False)
-                updated_query_params.append((query_name, new_val))
+                updated_query_params.append((query_name, new_val.encode('utf-8')))
             else:
                 # Make sure we're encoding Unicode strings down to their byte string
                 # representation so that `urlencode` can handle it.
@@ -286,11 +286,11 @@ class StaticContent(object):
 
         # Only encode this if told to.  Important so that we don't double encode
         # when working with paths that are in query parameters.
-        asset_path = asset_path.encode('utf-8')
         if encode:
+            asset_path = asset_path.encode('utf-8')
             asset_path = quote_plus(asset_path, '/:+@')
 
-        return urlunparse(('', base_url.encode('utf-8'), asset_path, params, urlencode(updated_query_params), ''))
+        return urlunparse(('', base_url, asset_path, params, urlencode(updated_query_params), ''))
 
     def stream_data(self):
         yield self._data

common/lib/xmodule/xmodule/contentstore/mongo.py

@@ -99,7 +99,9 @@ class MongoContentStore(ContentStore):
                               import_path=content.import_path,
                               # getattr b/c caching may mean some pickled instances don't have attr
                               locked=getattr(content, 'locked', False)) as fp:
-            if hasattr(content.data, '__iter__'):
+            # It seems that this code thought that only some specific object would have the `__iter__` attribute
+            # but the bytes object in python 3 has one and should not use the chunking logic.
+            if hasattr(content.data, '__iter__') and not isinstance(content.data, six.binary_type):
                 for chunk in content.data:
                     fp.write(chunk)
             else:

common/lib/xmodule/xmodule/modulestore/__init__.py

@@ -1298,7 +1298,8 @@ class ModuleStoreWriteBase(ModuleStoreReadBase, ModuleStoreWrite):
         result = defaultdict(dict)
         if fields is None:
             return result
-        cls = self.mixologist.mix(XBlock.load_class(category, select=prefer_xmodules))
+        classes = XBlock.load_class(category, select=prefer_xmodules)
+        cls = self.mixologist.mix(classes)
         for field_name, value in six.iteritems(fields):
             field = getattr(cls, field_name)
             result[field.scope][field_name] = value