Merge date fields in a more strong way

According to Mailchimp API we should "use YYYY-MM-DD to be safe" and now module just joins date array by "/" char, so this could lead us to not syncing date fields. I think, this issue is related to:

• #624326: Strange problem when syncing date field
• #1005950: Date field not merge

Providing patch which fixes sync problem for me, it constructs date string in a more strong way. Patch was made from 6.x-2.5.