date select within fields for with index

date_selects within fields_for don't add the index:

fields_for "user", :index => 1 do |ff|

ff.date_select :dob

end

renders as:

select id="user_dob_1i" name="user[dob(1i)]"

instead of:

select id="user_1_dob_1i" name="user[1][dob(1i)]"

This is my first attempt at a patch

Based on:

http://rails.lighthouseapp.com/p...

for some reason, it adds an index:

select name="post[456][updated_at(1i)]" id="post_456_updated_at_1i" index="456"

I guess that should be fixed?