Fix case where publicPath is a URL

[jodelamo]

Fixes #33

Not sure if others have experienced the exact same issue as me, but this "works on my machine".

To clarify; this part in replaceTag.js

if (prefix) {
  cssFilename = path.join(prefix, cssFilename);
}

Would change a publicPath containing a URL from this:

https://www.example.com/somepath/critical.css

To this (note the missing slash following the protocol):

https:/www.example.com/somepath/critical.css

[ghost]

Hello.
Yes, the same happens in my project and came up with the same fix too. I almost made a new PR, but luckily I found this one to bump instead.
I've tried this and it fixes the problem. I'm glad to help if there is anything else needed to have this PR merged in master.
Cheers

[numical]

Thanks!
I'll need to add tests for different types of public path before I can commit.
Will get onto this as soon as I can.
Or if you have time to add to the PR: example test is here.

[jodelamo]

@numical Tests added, courtesy of @gvitelli. 🙇

[numical]

Thanks all! Version 3.4.6 released with this PR.