BUG [JAVA] Generated java client code from openapi-generator-maven-plugin using okhttp-gson library failing to serialize RequestBody with contentType text/plain

Created by: prkumarm

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Have you provided a full/minimal spec to reproduce the issue?
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Description

Generated java client code from openapi-generator-maven-plugin using okhttp-gson library failing to serialize RequestBody with contentType text/plain.

When using httpok library(okhttp-gson) openapi-generator throws ApiException with message "Content type text/plain is not supported" for contentType text/plain.

I see below related mustache template code in serialize(Object obj, String contentType) method in ApiClient.mustache https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache

Is this a expected behavior? if so, what is the work around to serialize RequestBody of type text/plain

openapi-generator version

5.1.0

OpenApi generator dependency org.openapitools openapi-generator-maven-plugin 4.3.1

okhttp dependency - com.squareup.okhttp3 okhttp

OpenAPI declaration file content or url

N/A

Generation Details

Please refer steps defined below in "Steps to reproduce" section

Steps to reproduce

Step1 - Springfox generated contract with contentType text/plain

"/endPoint" : { "post" : { "tags" : [ "controller" ], "summary" : "summary", "operationId" : "operationId", "requestBody" : { "content" : { "application/json" : { "schema" : { "type" : "string" } }, "text/plain" : { "schema" : { "type" : "string" } } } }

Step 2: Generate client code using below Maven plugin for okhttp-gson library, org.openapitools openapi-generator-maven-plugin 4.3.1

Step3. Invoke rest endpoint using generated client code.

Output: Below error message is thrown by the generatened ApiClient class in serialize method.

"Content type text/plain is not supported" for contentType text/plain"