fix Bytes submission pb on POST, due to multiple call to field.process_form_value
for a same field.
In the case of file, stream.read() doesn't return file's data the second time
(since the cursor is at the end of the file).
Fix this by having a generic process_form_value method that cache field's value
in form.formvalues[field] and so _process_form_value is only called once.
You should then override that later method on custom fields.