Already encoded URLs are encoded twice

[NicolasMassart]

A bug introduced by #28 makes already encoded URLs encoded twice:

https://en.wikipedia.org/wiki/Glob_%28programming%29 becomes https://en.wikipedia.org/wiki/Glob_%2528programming%2529

To prevent this side effect, it's required to check if the URL is already encoded.
The simple way is to decode first. If the url is not encoded, the decoded will be the same, if it's encoded it will provide a clean version of the string.
The computation required to decode is very small and not even noticeable.

This was first reported as a markdown-link-check issue tcort/markdown-link-check#142 and it will be required to update the dependency once fixed.