fix get fallback

This fix a problem when `GET` method comes to `fallbackPjax`.
Usually in `GET` method, we tend to use 
```
{
    url: '/?a=1&b=2'
} 
```
rather than 
```
{
    url: '/', 
    data: 'a=1&b=2'
}
```
But any parameters written in `action` attribute of `<form>` element with `GET` method will be ignored. 
e.g. 
```
<form action="/?a=1&b=2" method='GET'>
```
is equivalent to 
```
<form action="/' method='GET'>
```
Thus we need to add these parameters using hidden `input`.