Remove redundant dependencies

lib/utils.js

@@ -1,10 +1,9 @@
 'use strict';
 
+const { types } = require('util');
 const crypto = require('crypto');
 const url = require('url');
 const formatDate = require('dateformat');
-const isNumber = require('is-number');
-const isString = require('is-string');
 
 const longDateRegExp = /^(\d{4})(\d{2})(\d{2})T(\d{2})(\d{2})(\d{2})Z$/;
 
@@ -119,7 +118,7 @@ class Utils {
   }
 
   static isValidHeaderValue(value) {
-    return isNumber(value) || isString(value);
+    return typeof value === 'number' || typeof value === 'string' || types.isStringObject(value);
   }
 
   static parseUrl(urlToParse, parseQueryString, slashesDenoteHost) {

package.json

@@ -55,8 +55,6 @@
     "tape": "^4.17.0"
   },
   "dependencies": {
-    "dateformat": "4.6.3",
-    "is-number": "7.0.0",
-    "is-string": "1.0.7"
+    "dateformat": "4.6.3"
   }
 }