fix: avoid mutating original rules array

close #1286

Author: yyx990803

Diff for: lib/plugin.js

@@ -26,7 +26,7 @@ class VueLoaderPlugin {
     }
 
     // get a hold of the raw rules
-    const rawRules = compiler.options.module.rules
+    const rawRules = compiler.options.module.rules.slice()
     // use webpack's RuleSet utility to normalize user rules
     const rawNormalizedRules = new RuleSet(rawRules).rules
 
@@ -127,6 +127,9 @@ class VueLoaderPlugin {
         return parsed.vue != null
       }
     })
+
+    // replace original rules
+    compiler.options.module.rules = rawRules
   }
 }