Fix path rewrite for pages with paths not ending with '/'

[jakubrohleder]

Today I started playing with this library a little and noticed, that for pages with paths, that does not end with '/' the static files paths are rewritten in the wrong way.

E.g. for page :

---
title: Hello World
date: "2015-05-01T22:12:03.284Z"
layout: post
path: "/hello-world"

---

![Chinese Salty Egg](./salty_egg.jpg)

Path of salty_egg.jpg will be rewritten into public/hello-worldsalty_egg.jpg instead of public/hello-world/salty_egg.jpg.

This simple patch fixes that issue.

[michaeljdeeb]

This seems to solve the problem and take care of the cases I was worried about in #223. Awesome!

[KyleAMathews]

So this seems great. I assume you've setup your server so it'll serve urls lacking trailing slashes correctly?

[jakubrohleder]

@KyleAMathews To be true I haven't done much. Just created extra directory /blog, copied index.html and post's directories there. Then I removed trailing slashes from post's front matter.

The only thing that was not working were images. With this PR everything seems to work fine with gatsby develop as well as on gh-pages.

[jlengstorf]

Hiya @jakubrohleder! 👋

This is definitely late, but on behalf of the entire Gatsby community, I wanted to say thank you for being here.

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