improve removeDuplicateSlashes

[Uzlopak]

If we replace the current implementation of removeDuplicateSlashes with the following:

const RemoveDuplicateSlashesRE = /\/\/+/g
function removeDuplicateSlashes (path) {
  return (path.indexOf('//') === -1 && path) || path.replace(RemoveDuplicateSlashesRE, '/')
}

then the case withoout duplicate slashes gets from about 4,284,328 ops/sec up to 5,419,257 ops/sec. The case that we have some duplicate slashes goes from 2,483,375 ops/sec to 2,292,004 ops/sec.

[ivan-tymoshenko]

Looks like it's worth it. Go for it.