Added fix for basic relative URLs and support variables in servers URLs

[udamir]

I don't want to put a specific url of server in OpenAPI file, as it can be used in different environments, so I use the following:

openapi: '3.0.2'
servers:
  - url: /api

On init method I get the error:

TypeError [ERR_INVALID_URL]: Invalid URL: /api
    at onParseError (internal/url.js:257:9)
    at new URL (internal/url.js:333:5)

[udamir]

This resolves #59

[kobik]

thanks for the PR @udamir!

please see my comments

[goldsziggy]

+1 on this fix

[kibertoad]

CI is passing locally, merging.

[kibertoad]

@kobik Could you please merge this?

[udamir]

What I need to do to pass ci/circleci checks?

[goldsziggy]

Bump on this change -- anything blocking it besides a review? I can add a commit if needed.

[kibertoad]

@udamir @goldsziggy This still needs tests, and also conflicts need to be resolved now that https://github.com/PayU/api-schema-builder/pull/67/files is merged, which I believe will be superseded by this PR - but you can definitely reuse the tests that were created for that PR.