Only extract value meta when element is not a form, closes phoenixf…

…ramework#2680

`extractMeta` receives a "root element".

When this element is an input, we want to retrieve the current value.

When the element is a form, `form.key` will look up any inputs in the
form with the name "key". If an input has the name "value", then we would
retrieve the input element *as the value*, instead of getting the value
of an input.

We explicitly avoid getting the value of a form to avoid this edgecase.

At time of writing, the value attribute is valid on button, data, input,
li, meter, option, progress and param tags.

assets/js/phoenix_live_view/view.js

@@ -849,7 +849,7 @@ export default class View {
       let name = el.attributes[i].name
       if(name.startsWith(prefix)){ meta[name.replace(prefix, "")] = el.getAttribute(name) }
     }
-    if(el.value !== undefined){
+    if(el.tagName !== "FORM" && el.value !== undefined){
       if(!meta){ meta = {} }
       meta.value = el.value