Fix overwrite middleware if proxy is String

Author: retyui

src/BrowserSyncDevHotWebpackPlugin.js

@@ -90,6 +90,15 @@ class BrowserSyncDevHotWebpackPlugin extends EventEmitter {
         this.browserSyncURLUI = null;
         this.browserSyncURLUIExternal = null;
 
+        if (
+            this.options.browserSync &&
+            typeof this.options.browserSync.proxy === "string"
+        ) {
+            const target = this.options.browserSync.proxy;
+
+            this.options.browserSync.proxy = { target };
+        }
+
         this.options.browserSync = merge.all([
             {
                 proxy: {