only infer scss/sass loader when no explicit loaders have been specified (fix #790)

Author: yyx990803

lib/loader.js

@@ -203,11 +203,13 @@ module.exports = function (content) {
         scoped: !!scoped,
         hasInlineConfig: !!query.postcss
       }) + '!'
-      // normalize scss/sass
-      if (lang === 'sass') {
-        lang = 'sass?indentedSyntax'
-      } else if (lang === 'scss') {
-        lang = 'sass'
+      // normalize scss/sass if no specific loaders have been provided
+      if (!loaders[lang]) {
+        if (lang === 'sass') {
+          lang = 'sass?indentedSyntax'
+        } else if (lang === 'scss') {
+          lang = 'sass'
+        }
       }
     }