@base prepend with a path, removes last folder if no end "/"

[ethieblin]

If we expand a URI with "@base" not ending with a "/", we get a URI missing the last path fragment.
For example
"@base":"http://example.com/loutre" with"@id":"toto" gives http://example.com/toto when we would expect http://example.com/loutre/toto. If "@base" ends with a "/" it works fine.

Playground expansion without "/"

Playground expansion with "/"

This issue is only on URI expansion, but the behaviour is different on URI compaction : a "@base" with or without "/" will give the same URI fragment.
This will be a problem when round-tripping because the input URI will change after re-expansion.

Input: "@id":"http://example.com/loutre/toto"
Compaction with "@base":"http://example.com/loutre" gives

"@context": {
    "@base": "http://example.com/loutre"
  },
  "@id": "toto"

Which re-expanded gives "http://example.com/toto".

Playground compaction, "@base" without "/"

Proposed fix: ethieblin#2