fix: server side Set-Cookie always set an array. (#367)

Author: sullivanpt

Diff for: lib/core/storage.js

@@ -226,14 +226,8 @@ export default class Storage {
       document.cookie = serializedCookie
     } else if (process.server && this.ctx.res) {
       // Send Set-Cookie header from server side
-      const prev = this.ctx.res.getHeader('Set-Cookie')
-      let value = serializedCookie
-      if (prev) {
-        value = Array.isArray(prev)
-          ? prev.concat(serializedCookie)
-          : [prev, serializedCookie]
-      }
-      this.ctx.res.setHeader('Set-Cookie', value)
+      const prevCookies = this.ctx.res.getHeader('Set-Cookie')
+      this.ctx.res.setHeader('Set-Cookie', [].concat(prevCookies, serializeCookie))
     }
 
     return value