Take "" and "/" as equivalent namespaces on server

On server, `.of(name)` should use the same key in `.nsps` object for "" and "/"

Important to use `String(name)` and `===` to keep out keys that don't cast to ""
Added relevant tests

"" and "/" namespaces are checked for equivalence on server and client
Update index.js
testing for equivalence of namespaces starting with / or without
Merge pull request #1729 from anupbishnoi/patch-1

Take "" and "/" as equivalent namespaces on server