Inconsistent functionality in date_helper.rb

I was working on some doc patches for the date/datetime helpers when I found out that there are some inconsistencies. IMO, all date helpers (both, object related and standard ones) should support the same options to define separators. This applies for all compound helpers - i.e. date/datetime/time_select and select_date/datetime/time.

Code examples:

# standard:
select_date(date, :date_separator => ' / ') # should work, but doesn't
select_datetime(date, :date_separator => ' / ') # should work AND is documented, but doesn't work
select_datetime(date, :datetime_separator => ' , ') # works, but isn't documented
select_datetime(date, :time_separator => ' - ') # should work, but doesn't
select_time(time, :time_separator => ' - ') # works

# object related:
date_select(object, method, :date_separator => ' / ') # should work, but doesn't
datetime_select(object, method, :datetime_separator => ' , ', :date_separator => ' / ', :time_separator => ' - ') # should work, but doesn't
time_select(object, method, :time_separator => ' - ') # should work, but doesn't

It's nothing big and it seems it's hardly ever used (otherwise, someone would have complained already) but I think it should be fixed nevertheless because it makes the date helper API inconsistent.

I've attached the failing tests for now and I'll try to come up with a patch in a day or two.