[FIX] orm: As float fields are stateful (the .digit attribute depend of the...

[FIX] orm: As float fields are stateful (the .digit attribute depend of the database (decimal_precision)), registry model instances need their owm copy.

lp bug: https://launchpad.net/bugs/929483 fixed

bzr revid: chs@openerp.com-20140116153708-061aq2k0n1qsb2n3