Only remove null and undefined query params

Right now Ziggy is stripping query params with empty strings and zeros (falsey values). Since those can be valid query params, this change updates it to only remove null and undefined values.

Fixes #157 and #141

src/js/route.js

@@ -101,7 +101,7 @@ class Router extends String {
         let queryString = '?';
 
         Object.keys(this.queryParams).forEach(function(key, i) {
-            if (this.queryParams[key]) {
+            if (this.queryParams[key] !== undefined && this.queryParams[key] !== null) {
                 queryString = i === 0 ? queryString : queryString + '&';
                 queryString += key + '=' + encodeURIComponent(this.queryParams[key]);
             }