[IMP] raise an error accessing an attribute on a recordset containing more than one record
[FIX] a typo in previous commit
[IMP] assertion on getter setter of recordset if it holds multiple records.
An exception will be raised in order to ensure consitent behavior and avoid bug due to non explicit behavior.
[FIX] base: be explicit when computing the res_currency date field
[FIX] base: missing case where rate_ids is empty
[IMP] adds new test to validate setter and getter behavior on recordset
Typo
Prevent crash due to API consolidation

Courtesy of @florentx
[MERGE] branches 'florentx-master-apiculture-fxi' and 'nbessi-assert-on-len-nbi' into master-apiculture

Both branches change the behavior of the field getter and setter:
 - one can get a field on a null or single record only
 - one can set a field on a single record only