Blog post creation does not properly handle dot anymore

To reproduce:

• go to the blog home page
• enter a post title with a do, for example "a.b"
• click Create

-> the generated URL is wrong: the "." became a "/"

Regression caused by BLOG-160 but it was only working by luck before because the "." was removed but the reference never really been properly created as shown on https://github.com/xwiki-contrib/application-blog/blob/0b35472bfb72aa0bf4f19eb4bf4283182e6a10cc/application-blog-ui/src/main/resources/Blog/CreatePost.xml#L94. It should use document reference API to serialize an accurate one instead of basic concatenation.