Basepath crops a letter of basepath when not using trailing slash

[dxlbnl]

I've added the basepath to the server config, and when calling sapper export --basepath blog
It's raising some 404 errors:

> Received 404 response when fetching /blo/
> Received 404 response when fetching /blo/blog

It does work when adding a trailing slash to basepath sapper export --basepath blog/

[Rich-Harris]

Thanks — fixed in 0.13.6