feat: respect Encoding Object while building requests (#9105)

This change fixes both:

1. making multipart/form-data requests with content-type
   header for every individual boundary
2. generating correct CURL command for multipart/form-data
   request, allowing specifying content-type header for every
   individual boundary

Refs #4826
Refs #5356

test/unit/core/curlify.js

@@ -1,6 +1,7 @@
 import Im from "immutable"
 import { requestSnippetGenerator_curl_bash as curl } from "core/plugins/request-snippets/fn.js"
 import win from "core/window"
+import { fromJSOrdered } from "core/utils"
 
 describe("curlify", function () {
 
@@ -200,25 +201,50 @@ describe("curlify", function () {
     expect(curlified).toEqual("curl -X 'POST' \\\n  'http://example.com' \\\n  -H 'content-type: multipart/form-data' \\\n  -F 'id=123' \\\n  -F 'file=@file.txt;type=text/plain'")
   })
 
-  it("should print a curl with object formData and file", function () {
-    let file = new win.File([""], "file.txt", { type: "text/plain" })
+  it("should print a curl with formData containing JSON and file", async function () {
+    /**
+     * Specialized sub-class of File class, that only
+     * accepts string data and retain this data in `data`
+     * public property throughout the lifecycle of its instances.
+     *
+     * This sub-class is exclusively used only when Encoding Object
+     * is defined within the Media Type Object (OpenAPI 3.x.y).
+     *
+     * Instances of a similar sub-class are produced by swagger-client request builder.
+     */
+    class FileWithData extends win.File {
+      constructor(data, name = "", options = {}) {
+        super([data], name, options)
+        this.data = data
+      }
+
+      valueOf() {
+        return this.data
+      }
+
+      toString() {
+        return this.valueOf()
+      }
+    }
+
+    let file = new win.File(["data"], "file.txt", { type: "text/plain" })
+    let optionsJSON = JSON.stringify({ some_array: ["string"], max_bar: 300 })
+    let options = new FileWithData(optionsJSON, "", { type: "application/json;charset=utf-8" })
+
+    let formData = new win.FormData()
+    formData.set("options", options)
+    formData.set("file", file)
 
     let req = {
       url: "http://example.com",
       method: "POST",
       headers: { "content-type": "multipart/form-data" },
-      body: {
-        options: JSON.stringify({
-          some_array: ["string"],
-          max_bar: 300,
-        }),
-        file
-      }
+      body: formData,
     }
 
-    let curlified = curl(Im.fromJS(req))
+    let curlified = curl(fromJSOrdered(req))
 
-    expect(curlified).toEqual(`curl -X 'POST' \\\n  'http://example.com' \\\n  -H 'content-type: multipart/form-data' \\\n  -F 'options={"some_array":["string"],"max_bar":300}' \\\n  -F 'file=@file.txt;type=text/plain'`)
+    expect(curlified).toEqual(`curl -X 'POST' \\\n  'http://example.com' \\\n  -H 'content-type: multipart/form-data' \\\n  -F 'options={"some_array":["string"],"max_bar":300};type=application/json;charset=utf-8' \\\n  -F 'file=@file.txt;type=text/plain'`)
   })
 
   it("should print a curl without form data type if type is unknown", function () {