restore breaking changes, add depd message

lib/response.js

@@ -18,6 +18,7 @@ const assert = require('assert');
 const extname = require('path').extname;
 const vary = require('vary');
 const only = require('only');
+const deprecate = require('depd')('koa');
 
 /**
  * Prototype.
@@ -240,6 +241,10 @@ module.exports = {
   /**
    * Perform a 302 redirect to `url`.
    *
+   * The string "back" is special-cased
+   * to provide Referrer support, when Referrer
+   * is not present `alt` or "/" is used.
+   *
    * Examples:
    *
    *    this.redirect('/login');
@@ -249,8 +254,13 @@ module.exports = {
    * @api public
    */
 
-  redirect(url) {
+  redirect(url, alt) {
     // location
+    if ('back' == url) {
+      deprecate('Special-cased string "back" through redirect will be removed in v3,' +
+                'consider migrating usage to ctx.back() instead.')
+      url = this.ctx.get('Referrer') || alt || '/';
+    }
     this.set('Location', url);
 
     // status

test/response/redirect.js

@@ -12,6 +12,34 @@ describe('ctx.redirect(url)', () => {
     assert.equal(ctx.status, 302);
   });
 
+  describe('with "back"', () => {
+    it('should redirect to Referrer', () => {
+      const ctx = context();
+      ctx.req.headers.referrer = '/login';
+      ctx.redirect('back');
+      assert.equal(ctx.response.header.location, '/login');
+    });
+
+    it('should redirect to Referer', () => {
+      const ctx = context();
+      ctx.req.headers.referer = '/login';
+      ctx.redirect('back');
+      assert.equal(ctx.response.header.location, '/login');
+    });
+
+    it('should default to alt', () => {
+      const ctx = context();
+      ctx.redirect('back', '/index.html');
+      assert.equal(ctx.response.header.location, '/index.html');
+    });
+
+    it('should default redirect to /', () => {
+      const ctx = context();
+      ctx.redirect('back');
+      assert.equal(ctx.response.header.location, '/');
+    });
+  });
+
   describe('when html is accepted', () => {
     it('should respond with html', () => {
       const ctx = context();