[FIX] expression.py: when searching on a datetime field with an argument formated as a date, conversion should happen _also_ for the '=' operator. This omission was causing an infinite loop beacuse we were pushing the exact same leaf on the stack without any modification
[IMP] base, currency: second attempt to include this patch (previously added in revision 4982 then reverted in revision 4984 because of a bug in the server that was making an infinite loop -which has been fixed in previous revision-). Updated the type of name field of res.currency.rate in <datetime> instead of <date> for those who need to update the rate of currencies several times per day