Laravel parsing a nested serialized query string

I'm trying to pass a nested set of query params to my Laravel 5.1 API. I'm using $httpParamSerializer(params) the serialize the object, which seems to do it correctly as the GET looks like:

GET "http://ift.tt/29v3V63"

Yet, when I return the $request from Laravel, it shows only one of the properties:

Object {category: "Consent Forms"}

Why is this?

Angular Frontend:

        var filters = {
            category : ['Anatomy', 'Consent Forms']
        };

        Category.getCategories(filters).then(function(res) {
            // Object {category: "Consent Forms"}
            console.log('CATEGORIES', res.data);

Angular HTTP Request:

        getCategories: function(params) {
            var qs = $httpParamSerializer(params);
            return $http({
                method: 'GET',
                url: url + ver + '/categories?' + qs,
                headers: Auth.getOAuthHeader(),
                cache: true
            });
        },

Laravel:

public function getCategories(Request $request) {
    $input = $request->all();
    return $input;

via Chebli Mohamed