add ctx.host= delegate

docs/api/context.md

@@ -119,6 +119,7 @@ throw err;
   - `ctx.querystring=`
   - `ctx.length`
   - `ctx.host`
+  - `ctx.host=`
   - `ctx.fresh`
   - `ctx.stale`
   - `ctx.socket`

lib/context.js

@@ -275,6 +275,14 @@ module.exports = {
     return this.request.host;
   },
 
+  /**
+   * Delegate to Request#host=.
+   */
+
+  set host(val) {
+    this.request.host = val;
+  },
+
   /**
    * Delegate to Request#fresh.
    */