Make posted data available to template tag. (bigskysoftware#835)

Demo code:

```html
<form hx-post="/form?token=123" hx-target="#result">
    <input name="name" value="John Smith" />
    <select name="role">
        <option selected value="admin">Admin</option>
        <option selected value="author">Author</option>
        <option selected value="guest">Guest</option>
    </select>
    <button type="submit">Submit</button> 
</form>

<output id="result"></output>

<template url="/form">
    <div>${JSON.stringify(params)}</div>
</template>
```

www/js/demo/it.js

@@ -44,7 +44,7 @@ function initMockRequests() {
                     {dynamicResponseModFn:
                             function(request, response, parameters) {
                                 console.log(request, response, parameters)
-                                return interpolate(elt.innerHTML, parameters);
+                                return interpolate(elt.innerHTML, { ...parameters, ...Object.fromEntries(new URLSearchParams(request)) });
                             },
                     usePathnameForAllQueries: true});
             }