fix: lack of trailing slash on publicPath string option

src/loader.js

@@ -40,7 +40,9 @@ export function pitch(request) {
   const childFilename = '*'; // eslint-disable-line no-path-concat
   const publicPath =
     typeof query.publicPath === 'string'
-      ? query.publicPath
+      ? query.publicPath.endsWith('/')
+        ? query.publicPath
+        : `${query.publicPath}/`
       : typeof query.publicPath === 'function'
       ? query.publicPath(this.resourcePath, this.rootContext)
       : this._compilation.outputOptions.publicPath;

test/cases/publicpath-trailing-slash/expected/main.css

@@ -0,0 +1,2 @@
+body { background: red; background-image: url(/static/img/cd0bb358c45b584743d8ce4991777c42.svg); }
+

test/cases/publicpath-trailing-slash/index.js

@@ -0,0 +1 @@
+import './style.css';

test/cases/publicpath-trailing-slash/style.css

@@ -0,0 +1 @@
+body { background: red; background-image: url(./react.svg); }

test/cases/publicpath-trailing-slash/webpack.config.js

@@ -0,0 +1,34 @@
+const Self = require('../../../');
+
+module.exports = {
+  entry: './index.js',
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          {
+            loader: Self.loader,
+            options: {
+              publicPath: '/static/img'
+            }
+          },
+          'css-loader',
+        ],
+      }, {
+        test: /\.(svg|png)$/,
+        use: [{
+          loader: 'file-loader',
+          options: {
+            filename: '[name].[ext]'
+          }
+        }]
+      }
+    ],
+  },
+  plugins: [
+    new Self({
+      filename: '[name].css',
+    }),
+  ],
+};