fix: use Node 6 compatible features

lib/index.js

@@ -1,4 +1,5 @@
-const { URLSearchParams } = require('url')
+const querystring = require('querystring')
+const url = require('url')
 
 class LogFilter {
   constructor (req, filter) {
@@ -20,18 +21,17 @@ class LogFilter {
       return this.req.url
     }
 
-    let parts = this.req.url.split('?')
+    let parsedUrl = url.parse(this.req.url, true)
 
-    let path = parts[0]
-    let searchParams = new URLSearchParams(parts[1])
+    let searchParams = parsedUrl.query
 
     this.filter.forEach(key => {
-      if (searchParams.get(key)) {
-        searchParams.set(key, '[FILTERED]')
+      if (searchParams[key]) {
+        searchParams[key] = '[FILTERED]'
       }
     })
 
-    return `${path}?${searchParams}`
+    return `${parsedUrl.pathname}?${querystring.encode(searchParams)}`
   }
 }