Exposed forms with checkboxes don't work correctly if the checkbox value is in the URL

1. Create a new view (page display) based on a content type that has a field that references another content type
2. Add a filter to that entity reference field and expose it like shown in the screenshot
3. In the advanced section, set it to Use Ajax: True
4. Save the form

Now the page will have an exposed filter that is just a single checkbox. Since the form uses AJAX you can select or deselect the checkbox and submit and things will work as expected. However, the value for that checkbox could also be passed in through the URL if you know what to pass. For example: `/test-view?network[1]=1`. That would set the checkbox to be default selected. However, now if you unselect the checkbox and click submit, you'll see that it gets re-checked.

I've tracked this down to the ViewsAjaxController ajaxView method combining POST and GET values. The AJAX request sends all values through a POST, and the GET will have the values that are in the URL bar. This is not an issue for text fields or select dropdowns, because the values for those fields always get POSTed. So combining POST and GET values with the POST values having preference will work fine. However, with a checkbox, when the checkbox is unselected, no value is passed in POST, so it'll use what is in GET, which is saying that it is still selected.

I am not sure if there is a reason to join the POST and GET values together, or if it would be fine to just use POST values if they are provided, otherwise use GET. I will provide a possible patch to address it that way (knowing it might not be ideal). Another option would be to ensure that a blank value for unchecked checkboxes gets POSTed on AJAX submits.