Integer Multipart Param: "Body may not be null"

[turtlegrammar]

Multipart params cannot be numbers:

(clj-http.client/post "http://localhost" {:multipart [{:name "number" :content 1}]})
;; => IllegalArgumentException Body may not be null  org.apache.http.util.Args.notNull (Args.java:54)

In clj-http.mutipart.create-multipart-entity, the type of (:content {:name "number" :content 1}) does not match any predicates in the cond clause, so part is bound to nil, which leads to the opaque error "Body may not be null."

Is there a reason why there isn't an else clause where things that aren't strings/streams/files/byte-arrays are stringified and treated as strings? At least, a clearer error would be helpful.

[dakrone]

I see this issue, I think the create-multipart-entity method should throw an exception on a nil part. Would you like to open a PR for this?