Expose factory traits and some clippy fixes (#983)

src/types/form.rs

@@ -192,6 +192,7 @@ pub struct UrlEncoded<U> {
 
 impl<U> UrlEncoded<U> {
     /// Create a new future to URL encode a request
+    #[allow(clippy::borrow_interior_mutable_const)]
     pub fn new(req: &HttpRequest, payload: &mut Payload) -> UrlEncoded<U> {
         // check content type
         if req.content_type().to_lowercase() != "application/x-www-form-urlencoded" {

src/types/json.rs

@@ -298,6 +298,7 @@ where
     U: DeserializeOwned + 'static,
 {
     /// Create `JsonBody` for request.
+    #[allow(clippy::borrow_interior_mutable_const)]
     pub fn new(
         req: &HttpRequest,
         payload: &mut Payload,

src/types/payload.rs

@@ -298,6 +298,7 @@ pub struct HttpMessageBody {
 
 impl HttpMessageBody {
     /// Create `MessageBody` for request.
+    #[allow(clippy::borrow_interior_mutable_const)]
     pub fn new(req: &HttpRequest, payload: &mut dev::Payload) -> HttpMessageBody {
         let mut len = None;
         if let Some(l) = req.headers().get(&header::CONTENT_LENGTH) {