fix a bug that post.permalink error when config.relative_link is true

[tcatche]

When set relative_link to true in config.yml, the post.permalink will incrrect.

For example:
When config.url is http://yoursite.com/ and the post's path is 2017/09/a-post ,the result will http://yoursite.com2017/09/a-post . It misses a separator / after the site url.

This pull request aims to repair this problem.

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.0007%) to 97.165% when pulling c2437d8 on tcatche:master into 9ad6c47 on hexojs:master.

[coveralls]

Coverage increased (+0.0007%) to 97.165% when pulling 6156391 on tcatche:master into 9ad6c47 on hexojs:master.

[NoahDragon]

LGTM.

[NoahDragon]

I will keep the PR unmerged, hope someone uses relative_url can give it a try.

[JLHwung]

Thank you so much for you contribution. Hexo can't be rock without contributors like you.