Prepare Frame Form Submission fetch headers

Closes hotwired#86
Closes hotwired#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 introduces a readonly `FetchRequestHeaders` property created at
constructor-time so that its values can be mutated prior to the
request's submission.

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

src/core/frames/frame_controller.ts

@@ -144,6 +144,8 @@ export class FrameController implements AppearanceObserverDelegate, FetchRequest
     if (this.formSubmission.fetchRequest.isIdempotent) {
       this.navigateFrame(element, this.formSubmission.fetchRequest.url.href)
     } else {
+      const { fetchRequest } = this.formSubmission
+      this.prepareHeadersForRequest(fetchRequest.headers, fetchRequest)
       this.formSubmission.start()
     }
   }

src/http/fetch_request.ts

@@ -42,13 +42,15 @@ 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
 
   constructor(delegate: FetchRequestDelegate, method: FetchMethod, location: URL, body: FetchRequestBody = new URLSearchParams) {
     this.delegate = delegate
     this.method = method
+    this.headers = this.defaultHeaders
     if (this.isIdempotent) {
       this.url = mergeFormDataEntries(location, [ ...body.entries() ])
     } else {
@@ -75,6 +77,7 @@ export class FetchRequest {
 
   async perform(): Promise<FetchResponse> {
     const { fetchOptions } = this
+    this.delegate.prepareHeadersForRequest?.(this.headers, this)
     dispatch("turbo:before-fetch-request", { detail: { fetchOptions } })
     try {
       this.delegate.requestStarted(this)
@@ -112,27 +115,19 @@ export class FetchRequest {
     }
   }
 
-  get isIdempotent() {
-    return this.method == FetchMethod.get
+  get defaultHeaders() {
+    return {
+      "Accept": "text/html, application/xhtml+xml"
+    }
   }
 
-  get headers() {
-    const headers = { ...this.defaultHeaders }
-    if (typeof this.delegate.prepareHeadersForRequest == "function") {
-      this.delegate.prepareHeadersForRequest(headers, this)
-    }
-    return headers
+  get isIdempotent() {
+    return this.method == FetchMethod.get
   }
 
   get abortSignal() {
     return this.abortController.signal
   }
-
-  get defaultHeaders() {
-    return {
-      "Accept": "text/html, application/xhtml+xml"
-    }
-  }
 }
 
 function mergeFormDataEntries(url: URL, entries: [string, FormDataEntryValue][]): URL {

src/tests/functional/form_submission_tests.ts

@@ -205,6 +205,14 @@ 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
@@ -298,6 +306,14 @@ 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]")
   }