Proxy 404 when using string shorthand

[cunc]

Describe the bug

Proxy 404 when using string shorthand

Reproduction

import { defineConfig } from 'vite'
import vue from '@vitejs/plugin-vue'

export default defineConfig({
  plugins: [vue()],
  server: {
    proxy: {
      '/frontend': 'http://example.com', // this made an 404 error when calling API, no error in terminal

      // below is OK
      '/frontend': {
        target: 'http://example.com'',
        changeOrigin: true
      }
    },
  }
})

System Info

• vite version: 2.0.0-beta.30
• Operating System: Big sur
• Node version: 15.6.0
• Package manager (npm/yarn/pnpm) and version: npm