Ensure _target is sent as "undefined" when it doesn't exist (#3731)

Fixes #3727.

In LV <= 1.0.5 we sent the `_target` as part of the url encoded form
values, but in 1.0.6 we switched to sending the meta values directly
in the form event to support `JS.push`'s value parameter for nested
values. Since the event itself is JSON encoded, this meant that any keys
with undefined value would be dropped by JS. To be backwards compatible,
we now ensure that the `_target` is always sent and restore the old
behavior of it being the string `"undefined"`, although that seems a bit
ugly. It is what it is now...

Author: SteffenDE

assets/js/phoenix_live_view/view.js

@@ -1212,7 +1212,14 @@ export default class View {
       type: "form",
       event: phxEvent,
       value: formData,
-      meta: {_target: opts._target, ...meta},
+      meta: {
+        // no target was implicitly sent as "undefined" in LV <= 1.0.5, therefore
+        // we have to keep it. In 1.0.6 we switched from passing meta as URL encoded data
+        // to passing it directly in the event, but the JSON encode would drop keys with
+        // undefined values.
+        _target: opts._target || "undefined",
+        ...meta
+      },
       uploads: uploads,
       cid: cid
     }

assets/test/view_test.js

@@ -276,6 +276,31 @@ describe("View + DOM", function(){
     view.pushInput(input, el, null, "validate", {_target: input.name, value: optValue})
   })
 
+  test("pushInput with nameless input", function(){
+    expect.assertions(4)
+
+    let liveSocket = new LiveSocket("/live", Socket)
+    let el = liveViewDOM()
+    let input = el.querySelector("input")
+    input.removeAttribute("name")
+    simulateUsedInput(input)
+    let view = simulateJoinedView(el, liveSocket)
+    let channelStub = {
+      push(_evt, payload, _timeout){
+        expect(payload.type).toBe("form")
+        expect(payload.event).toBeDefined()
+        expect(payload.value).toBe("_unused_note=&note=2")
+        expect(payload.meta).toEqual({"_target": "undefined"})
+        return {
+          receive(){ return this }
+        }
+      }
+    }
+    view.channel = channelStub
+
+    view.pushInput(input, el, null, "validate", {_target: input.name})
+  })
+
   test("getFormsForRecovery", function(){
     let view, html, liveSocket = new LiveSocket("/live", Socket)