Fix bug in dev-server when a proxyTable entry is a string (#965)

`options` can not be a `const` because it is modified if value is a string.

Author: schettino72

template/build/dev-server.js

@@ -51,7 +51,7 @@ app.use(hotMiddleware)
 
 // proxy api requests
 Object.keys(proxyTable).forEach(function (context) {
- const options = proxyTable[context]
+ let options = proxyTable[context]
  if (typeof options === 'string') {
  options = { target: options }
  }