Add Content-Type on Param-Type

Hello,

I use your gem on a project using Salesforce API.
I encountered a problem on the ParamPart. I need to specify the Content-Type of my body otherwise Salesforce can't parse it correctly.

I think it can be useful in other case so I add the feature.

I add an unit test in test_multipart.rb, simply called 'test_form_multipart_body_with_content_type'

Salesforce reference documentation :
http://www.salesforce.com/us/developer/docs/chatterapi/Content/intro_input.htm

Best Regards,
Vincent