[py3] Decode request's content before JSON loading it

In Python3, json.load() only accepts text content. So we decode
_cw.content's value using request's encoding before passing it to
json.load(). As a consequence of using .getvalue() on BytesIO (content
is a BytesIO), we no longer need to .seek() after reading the content.

views.py

@@ -107,8 +107,8 @@ class RqlIOController(Controller):
             contenttype = self._cw.get_header('Content-Type', raw=False)
             if (contenttype.mediaType, contenttype.mediaSubtype) == ('application', 'json'):  # noqa: E501
                 encoding = contenttype.params.get('charset', 'utf-8')
-                args = json.load(self._cw.content, encoding=encoding)
-                self._cw.content.seek(0, 0)
+                content = self._cw.content.getvalue().decode(encoding)
+                args = json.loads(content)
             else:
                 args = json.load(self._cw.form['json'][1])