convert session to dict before pickling

Session was constantly being pickled without converting to dict as it
should be.
Update tools.py

Fixed email date header not respecting the RFC2822 defined format

See:
https://groups.google.com/forum/#!topic/web2py/0Mb99H9kJOw
Revert "convert session to dict before pickling"

This reverts commit 36303e338d01e52f3ada9d2569d50106bfa6b921.
Merge pull request #383 from leonelcamara/master

Fix email date header format