Fix authentication when @scope:registry does not specify trailing slash

[KidkArolis]

This PR was triggered by new discussion on #1666.

I'm not 100% sold on this yet, I'll try to read through the npm source code to see what they do.

Which one of these cases are valid (cause this PR would break some of these):

# Standard setup - works today, both entries have trailing slash
@scoped:registry=https://foo.com/
//foo.com/:_authToken=xyz

# This PR addresses this case - does this work in npm-cli?
@scoped:registry=https://foo.com
//foo.com/:_authToken=xyz

# Is this possible? No trailing slashes
@scoped:registry=https://foo.com
//foo.com:_authToken=xyz

# I keep referring to these as trailing slashes, but really - they're paths
@scoped:registry=https://foo.com/custom/path
//foo.com/custom/path:_authToken=xyz

# And this?
@scoped:registry=https://foo.com/custom/path/
//foo.com/custom/path/:_authToken=xyz

# And this?
@scoped:registry=https://foo.com/custom/path
//foo.com/custom/path/:_authToken=xyz