Swagger codegen oAuth issue

Description

The OAUTH with basic authentication sends the headers to request the token in the body instead in the authorization header with bearer.

Swagger-codegen version

The latest from Maven.

Swagger declaration file content or url

We wanted to create a swagger client for java with the feign library. The problem is, that all the variables are send within the body to request the authentication from the auth server.
While it is needed to have those in the headers.

Command line used for generation

Generated with the Maven plugin.

Steps to reproduce

Generate client with oAuth and basic auth (double auth).
Then instantiate this client and try to get the client

Suggest a fix/enhancement

Exception in thread "main" feign.RetryableException
at io.swagger.client.auth.OAuth.updateAccessToken(OAuth.java:97)
at io.swagger.client.auth.OAuth.apply(OAuth.java:83)
at feign.SynchronousMethodHandler.targetRequest(SynchronousMethodHandler.java:158)
at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:88)
at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:76)
at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:103)
at com.sun.proxy.$Proxy5.welcomeUsingGET(Unknown Source)
at com.test.Main.main(Main.java:33)
Caused by: java.lang.NullPointerException
at io.swagger.client.auth.OAuth$OAuthFeignClient.execute(OAuth.java:177)
at org.apache.oltu.oauth2.client.OAuthClient.accessToken(OAuthClient.java:65)
at org.apache.oltu.oauth2.client.OAuthClient.accessToken(OAuthClient.java:55)
at org.apache.oltu.oauth2.client.OAuthClient.accessToken(OAuthClient.java:71)
at io.swagger.client.auth.OAuth.updateAccessToken(OAuth.java:95)
... 7 more

I guess that there needs to be a template change that places the beared in the authorization header instead inside the body.

Start with the generated OAuth flow in OAuth.java, especially the updateAccessToken, apply, and OAuthFeignClient.execute locations shown in the stack trace, and inspect the Maven generation templates that produce it. Reproduce a Java Feign client with OAuth and basic authentication; done means token-request credentials use the required authorization header and the reported NullPointerException no longer occurs.