As TextDocument can store this original value in data property (Ingestion Input)
or text_content property (UI Input).
content_md5 is computed against getData, then TextDocument instances must fallback
on getTextContent if data is empty.
getData usage is unified among all types of documents.

Additional Comments:
TextDocument._convert() still rely on text_content value as raw_data.
because if data exists, text_content also, as a base conversion of data.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@41849 20353a03-c40f-0410-a6d1-a30d3c3de9de