Don't assume fetchOptions has a body (#384)

The event handler in encodeMethodIntoRequestBody was failing when forms
were submitted using `method="GET"`, because it assumed the presence of
`fetchOptions.body`, and such forms don't have a body.

Checking for the presence of `body` before accessing it avoids the
crash, and the resulting console noise.

app/assets/javascripts/turbo.js

@@ -3936,7 +3936,7 @@ 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.get("_method") || form.getAttribute("method");
+      const method = submitter && submitter.formMethod || fetchOptions.body && fetchOptions.body.get("_method") || form.getAttribute("method");
       if (!/get/i.test(method)) {
         if (/post/i.test(method)) {
           fetchOptions.body.delete("_method");