Add Type Safety Guards to turbo/fetch_requests

Without this change, GET `<form>` submissions without a Fetch Options `{
body: }` raise the following:

```
Uncaught TypeError: s.body is null
    <anonymous> fetch_requests.js:10
    w turbo.es2017-esm.js:351
    requestStarted turbo.es2017-esm.js:770
    perform turbo.es2017-esm.js:520
    start turbo.es2017-esm.js:744
    formSubmitted turbo.es2017-esm.js:3369
```

Through the exercise of attempting to [port the `turbo/fetch_requests`
module to
TypeScript](#392 (comment)),
we've identified some potential edge cases in the algorithm that
determines a request's HTTP method.

Even if the migration to TypeScript doesn't come to fruition for some
time, those edge cases should be addressed sooner rather than later.

This commit adds more "type safety" motivated conditionals and guards to
make sure that values are present before overriding them.

app/assets/javascripts/turbo.js

@@ -4025,12 +4025,13 @@ function encodeMethodIntoRequestBody(event) {
   if (event.target instanceof HTMLFormElement) {
     const {target: form, detail: {fetchOptions: fetchOptions}} = event;
     form.addEventListener("turbo:submit-start", (({detail: {formSubmission: {submitter: submitter}}}) => {
-      const method = submitter && submitter.formMethod || fetchOptions.body && fetchOptions.body.get("_method") || form.getAttribute("method");
+      const body = isBodyInit(fetchOptions.body) ? fetchOptions.body : new URLSearchParams;
+      const method = determineFetchMethod(submitter, body, form);
       if (!/get/i.test(method)) {
         if (/post/i.test(method)) {
-          fetchOptions.body.delete("_method");
+          body.delete("_method");
         } else {
-          fetchOptions.body.set("_method", method);
+          body.set("_method", method);
         }
         fetchOptions.method = "post";
       }
@@ -4040,6 +4041,35 @@ function encodeMethodIntoRequestBody(event) {
   }
 }
 
+function determineFetchMethod(submitter, body, form) {
+  const formMethod = determineFormMethod(submitter);
+  const overrideMethod = body.get("_method");
+  const method = form.getAttribute("method") || "get";
+  if (typeof formMethod == "string") {
+    return formMethod;
+  } else if (typeof overrideMethod == "string") {
+    return overrideMethod;
+  } else {
+    return method;
+  }
+}
+
+function determineFormMethod(submitter) {
+  if (submitter instanceof HTMLButtonElement || submitter instanceof HTMLInputElement) {
+    if (submitter.hasAttribute("formmethod")) {
+      return submitter.formMethod;
+    } else {
+      return null;
+    }
+  } else {
+    return null;
+  }
+}
+
+function isBodyInit(body) {
+  return body instanceof FormData || body instanceof URLSearchParams;
+}
+
 addEventListener("turbo:before-fetch-request", encodeMethodIntoRequestBody);
 
 var adapters = {