Return bytes in EpubHtml's get_body_content

The method's documentation states that in python 3 it return bytes, but
was previously returning strings in some situations.

ebooklib/epub.py

@@ -352,7 +352,7 @@ def get_body_content(self):
         try:
             html_tree = parse_html_string(self.content)
         except:
-            return ''
+            return six.b('')
 
         html_root = html_tree.getroottree()
 
@@ -369,7 +369,7 @@ def get_body_content(self):
 
             return tree_str
 
-        return ''
+        return six.b('')
 
     def get_content(self, default=None):
         """