to setup default file

now we can setup default file to some thing like 'default.htm'. 
The default value is still 'index.html', same as the old version.
when session.save error should return error

1
Revert "when session.save error should return error"

This reverts commit 437f21a67de194ca467f4b2aec91e0018330bba1.
session: add long expires check and prevent set-cookie
Release 2.7.4