Update dal.py

I think It will be useful to update the table contents and to validate the data previously.

On the other hand, I think there is something weird in the update_or_insert definition (Class table) because it is not working properly on data updating.
fixed issue 1643:grid - export HTML - missing charset=UTF-8, problems with non-ASCII-chars; proposal, thanks Martin
pdfinvoice
session id null on cookieless client.
error on partition usage fixed
Merge pull request #177 from michele-comitini/session_id_fix

session id null on cookieless client.
reverted issue 1639:DAL, define_table and Field unpacked serialized attributes
Merge pull request #176 from mcabo/patch-1

Update dal.py
trying to trigget travis....
fixed env['QUERY_STRING'], thanks Anthony
fixed Encoding problem in 'format' from define_table on GAE, thanks Juan
solved problems with auth and cookie expire
fixed latest mdmdb problem?
Merge ssh://github.com/web2py/web2py