fixed bug for same name parameter
fix bug of  post 'multipart/form-data' request without file data
add content-type parameter for char-stream data in multipart/form-data

add content-type parameter for char-stream data in multipart/form-data &
fix bug of writing wrong last boundary
Add RequestParams#add

This is a shortcut for adding a parameter which can have many values; useful for communicating for something like Rails, where array parameters are non-unique.
DRY
doc