Prepare Frame Form Submission fetch headers

Closes #86
Closes #110

When submitting a Form that is within a `<turbo-frame>` or targets a
`<turbo-frame>`, ensure that the `Turbo-Frame` header is present.

Since the constructive-style
`FetchRequestDelegate.additionalHeadersForRequest()` was replaced by the
mutative style `FetchRequestDelegate.prepareHeadersForRequest()`, this
commit changes the `FetchRequest.headers` property to be `readonly` and
created at constructor-time so that its values can be mutated prior to
the request's submission.

Testing adds listeners for `turbo:before-fetch-request` and
`turbo:before-fetch-response` so that the event logs can drain.

Co-authored-by: tleish <tleish@users.noreply.github.com>

src/core/frames/frame_controller.ts

@@ -130,6 +130,8 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
       const frame = this.findFrameElement(element, submitter)
       frame.src = this.formSubmission.fetchRequest.url.href
     } else {
+      const { fetchRequest } = this.formSubmission
+      this.prepareHeadersForRequest(fetchRequest.headers, fetchRequest)
       this.formSubmission.start()
     }
   }

src/http/fetch_request.ts

@@ -42,6 +42,7 @@ export interface FetchRequestOptions {
 export class FetchRequest {
   readonly delegate: FetchRequestDelegate
   readonly method: FetchMethod
+  readonly headers: FetchRequestHeaders
   readonly url: URL
   readonly body?: FetchRequestBody
   readonly abortController = new AbortController
@@ -55,6 +56,7 @@ export class FetchRequest {
       this.body = body
       this.url = location
     }
+    this.headers = prepareHeadersForRequest(this)
   }
 
   get location(): URL {
@@ -116,23 +118,17 @@ export class FetchRequest {
     return this.method == FetchMethod.get
   }
 
-  get headers() {
-    const headers = { ...this.defaultHeaders }
-    if (typeof this.delegate.prepareHeadersForRequest == "function") {
-      this.delegate.prepareHeadersForRequest(headers, this)
-    }
-    return headers
-  }
-
   get abortSignal() {
     return this.abortController.signal
   }
+}
 
-  get defaultHeaders() {
-    return {
-      "Accept": "text/html, application/xhtml+xml"
-    }
+function prepareHeadersForRequest(fetchRequest: FetchRequest) {
+  const headers = { "Accept": "text/html, application/xhtml+xml" }
+  if (typeof fetchRequest.delegate.prepareHeadersForRequest == "function") {
+    fetchRequest.delegate.prepareHeadersForRequest(headers, fetchRequest)
   }
+  return headers
 }
 
 function mergeFormDataEntries(url: URL, entries: [string, FormDataEntryValue][]): URL {

src/tests/fixtures/test.js

@@ -35,4 +35,6 @@
   "turbo:frame-load",
   "turbo:frame-render",
   "turbo:frame-visit",
+  "turbo:before-fetch-request",
+  "turbo:before-fetch-response",
 ])

src/tests/functional/form_submission_tests.ts

@@ -273,6 +273,14 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.equal(htmlAfter, htmlBefore)
   }
 
+  async "test frame form submission within a frame submits the Turbo-Frame header"() {
+    await this.clickSelector("#frame form.redirect input[type=submit]")
+
+    const { fetchOptions } = await this.nextEventNamed("turbo:before-fetch-request")
+
+    this.assert.ok(fetchOptions.headers["Turbo-Frame"], "submits with the Turbo-Frame header")
+  }
+
   async "test invalid frame form submission with unprocessable entity status"() {
     await this.clickSelector("#frame form.unprocessable_entity input[type=submit]")
     await this.nextBeat
@@ -351,6 +359,14 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.equal(await this.pathname, "/src/tests/fixtures/one.html")
   }
 
+  async "test form submission targeting a frame submits the Turbo-Frame header"() {
+    await this.clickSelector('#targets-frame [type="submit"]')
+
+    const { fetchOptions } = await this.nextEventNamed("turbo:before-fetch-request")
+
+    this.assert.ok(fetchOptions.headers["Turbo-Frame"], "submits with the Turbo-Frame header")
+  }
+
   get formSubmitted(): Promise<boolean> {
     return this.hasSelector("html[data-form-submitted]")
   }