refactor: handle header error

Author: davidebianchi

lib/request.js

@@ -5,6 +5,7 @@
 const { Readable } = require('readable-stream')
 const util = require('util')
 const cookie = require('cookie')
+const assert = require('assert')
 
 const parseURL = require('./parseURL')
 
@@ -47,10 +48,8 @@ function Request (options) {
   const headers = options.headers || {}
   Object.keys(headers).forEach((field) => {
     const value = headers[field]
-    if (value === undefined) {
-      throw new Error('Invalid header value: invalid value "undefined" for header ' + field)
-    }
-    this.headers[field.toLowerCase()] = '' + headers[field]
+    assert(value !== undefined, 'invalid value "undefined" for header ' + field)
+    this.headers[field.toLowerCase()] = '' + value
   })
 
   this.headers['user-agent'] = this.headers['user-agent'] || 'lightMyRequest'