Fixed issues with Content-Type headers. If one is present it should be modified without adding a new one (which would be disregarded by the browser)

src/middleware/mod.rs

@@ -59,7 +59,14 @@ where
         let val = HeaderValue::from_str(self.mime_type.as_str()).expect("Invalid MimeType");
         Box::pin(async move {
             let mut res = fut.await?;
+            match res.headers_mut().get_mut(CONTENT_TYPE) {
+                Some(content_type) => {
+                    *content_type = val;
+                }
+                None => {
                     res.headers_mut().append(CONTENT_TYPE, val);
+                }
+            }
             Ok(res)
         })
     }