Fix multipart uploads with boolean params.

[rywall]

I am using a gem (ocr_space) that passes a boolean param to HTTParty.post. This started failing in v0.16 with the following error message:

TypeError: no implicit conversion of false into String
from /gems/httparty-0.16.0/lib/httparty/request/body.rb:40:in `+'

[jnunemaker]

I'm going to merge this. I have some hesitation adding the to_s as it could cover up passing in nil or other not serializable objects accidentally, but we'll see how it goes. :)