fix(index.js): fix path resolving for prefixed urls

[artptr]

There was an incorrect path resolution in case of mapping of a serving directory to a prefixed url.

The reason is the path.resolve() function doesn't join its arguments. And in case of prefixed url, the fp parameter contains a leading slash.

[coveralls]

Coverage remained the same at 100.0% when pulling 8990013 on artptr:fix-path-resolving into c9fec80 on tunnckoCore:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 8990013 on artptr:fix-path-resolving into c9fec80 on tunnckoCore:master.

[tunnckoCore]

Hmm. Strange. Good catch. I hate these filepath shits in node.

I'll merge when i get a bit more time, thanks 🎉

[tunnckoCore]

@artptr thanks and sorry for the delay!