refactor: use static insert option from constructor

Author: ScriptedAlchemy

src/index.js

@@ -101,7 +101,10 @@ class CssModuleFactory {
 class MiniCssExtractPlugin {
   constructor(options = {}) {
     validateOptions(schema, options, 'Mini CSS Extract Plugin');
-    const insert = options.insert ? options.insert.toString() : null;
+    this.options.insert
+     ? Template.asString([this.options.insert, 'insert(linkTag);'])
+     : 'var head = document.getElementsByTagName("head")[0]; head.appendChild(linkTag)
+    
     this.options = Object.assign(
       {
         filename: DEFAULT_FILENAME,
@@ -377,8 +380,6 @@ class MiniCssExtractPlugin {
                       ])
                     : '',
                   this.options.insert
-                    ? Template.asString([this.options.insert, 'insert(linkTag);'])
-                    : 'var head = document.getElementsByTagName("head")[0]; head.appendChild(linkTag)'
                 ]),
                 '}).then(function() {',
                 Template.indent(['installedCssChunks[chunkId] = 0;']),