posts: fix /posts/random?tags=... redirect.

Fix /posts/random?tags=touhou to redirect to /posts/1234?q=touhou instead of /posts/1234?tags=touhou.

Fixes the search bar on random posts not containing the current search.

app/controllers/posts_controller.rb

@@ -132,7 +132,7 @@ class PostsController < ApplicationController
     raise ActiveRecord::RecordNotFound if @post.nil?
     authorize @post
     respond_with(@post) do |format|
-      format.html { redirect_to post_path(@post, :tags => params[:tags]) }
+      format.html { redirect_to post_path(@post, q: params[:tags]) }
     end
   end