[BUG][Java] Generated okhttp-gson does not work with relative OAuth2 token URLs

Created by: sebastien-rosset

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Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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Description

The generated Java client SDK throws an IllegalArgumentException when the OAS document contains a OAuth2 security scheme and the token URL is a relative URL. I don't think there is anything in the OAS spec that states the URL must be an absolute URL.

Exception in thread "main" java.lang.IllegalArgumentException: Expected URL scheme 'http' or 'https' but no colon was found
	at okhttp3.HttpUrl$Builder.parse(HttpUrl.java:1332)
	at okhttp3.HttpUrl.get(HttpUrl.java:917)
	at okhttp3.Request$Builder.url(Request.java:165)
	at .....client.auth.OAuthOkHttpClient.execute(OAuthOkHttpClient.java:35)

openapi-generator version

master March 4th 2020

OpenAPI declaration file content or url

  securitySchemes:
    oAuth2:
      type: oauth2
      flows:
       clientCredentials:
          tokenUrl: /iam/token

Command line used for generation

Steps to reproduce

Related issues/PRs

Suggest a fix

No exception should be thrown. The generated client should use the server prefix URL if the token URL is relative. Use ApiClient.getBasePath() method.