Encode request body for MailChimp API 1.3 requests

According to MailChimp in this thread:

https://groups.google.com/forum/?fromgroups=#!topic/mailchimp-api-discuss/3Po1CDGMqJA

it sounds like they're expecting all JSON request bodies be URL-encoded.  This change has allowed us to successfully subscribe email addresses with "+" characters.  I'm not sure what the other implications are as I'm only familiar with a small subset of the MailChimp API (and node-mailchimp).