Submit method="GET" must ignore query string

In a HTML form GET submit, the URL must ignore query string.

```html

Submit

```

```python
browser = RoboBrowser(parser="lxml")
browser.open("http://localhost:8000/")

form = browser.get_form("form")
form["field"] = "value"
browser.submit_form(form)
# browser.url == "http://localhost:8000/?field=value"

form = browser.get_form("form")
form["field"] = "other"
browser.submit_form(form)
# browser.url == "http://localhost:8000/?field=value&field=other"
```

The url must be `http://localhost:8000/?field=other`, removing previous query string.

Start by reproducing the two browser.submit_form(form) calls from the issue and trace how GET form submission builds the URL. Verify that a second submission replaces the existing query string rather than appending to it, with the final URL matching the example.

Written by the indexing model from the issue text.