fix(index): copy loader options before modifying (#326)

Author: clydin

lib/index.js

@@ -40,7 +40,7 @@ const SyntaxError = require('./Error')
  * @return {cb} cb      Result
  */
 module.exports = function loader (css, map) {
-  const options = loaderUtils.getOptions(this) || {}
+  const options = Object.assign({}, loaderUtils.getOptions(this))
 
   validateOptions(require('./options.json'), options, 'PostCSS Loader')
 
@@ -51,7 +51,7 @@ module.exports = function loader (css, map) {
 
   Promise.resolve().then(() => {
     const length = Object.keys(options)
-      .filter((option) =>  {
+      .filter((option) => {
         switch (option) {
           // case 'exec':
           case 'ident':