Kibana server doesn't redirect URL with trailing slashes

[mshustov]

Kibana version: master

Steps to reproduce:

1. run Kibana.
2. navigate to a registered route with trailing slash at the end. for example http://localhost:5601/basePath/api/kibana/scripts/languages/ .

Expected behavior:
The server redirects to a correct route http://localhost:5601/basePath/api/kibana/scripts/languages.

Actual behavior
The server redirects to invalid route http://localhost:5601/basePath//api/kibana/scripts/languages.

Redirection logic can be broken for a long time https://github.com/restrry/kibana/blob/cdee299d0a58de9c5e98ee36a35bcd6e66265887/src/legacy/server/http/index.js#L96-L113
Don't know if it's something we want to maintain. If yes, we should cover it with integration tests.

[elasticmachine]

Pinging @elastic/kibana-platform

[Ghost---Shadow]

Can reproduce in 6.x.x and 7.x.x