[JAVA][library: vertx] generated client fails to deserialize text/plain endpoints

Description:

Java-vertx client fails to deserialize text/plain endpoints due attempt to deserialize non-Json content:

Json.decodeValue(httpResponse.body(), returnType);

Swagger-codegen version:

2.4.7.

Swagger declaration file content or url

/metrics:
    get:
      tags:
      - "metrics"
      operationId: "getMetrics"
      produces:
      - text/plain
      responses:
        200:
          description: "Metrics"
          schema:
            type: "string"

Command line used for generation

java -DsupportingFiles -Dmodels -DmodelDocs=false -DmodelTests=false -Dapis -jar ./swagger-codegen-cli-2.4.7.jar \
generate -i swagger-api.yaml --lang java --additional-properties basePackage=service \
--api-package api --model-package model --additional-properties configPackage=configuration \
--additional-properties java8=true --additional-properties dateLibrary=java8 \
--additional-properties sourceFolder=generated-sources --additional-properties library=vertx -o src/main;

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

add another type check into if-else sieve (ApiClient.buildResponseHandler), e.g.:

if ("byte[]".equals(returnType.getType().toString())) {
    resultContent = (T) httpResponse.body().getBytes();
} else if (AsyncFile.class.equals(returnType.getType())) {
    handleFileDownload(httpResponse, handler);
    return;
} else {
    if ("java.lang.String".equals(returnType.getType().getTypeName())) {
        resultContent = (T) new String(httpResponse.body().getBytes());
    } else {
        resultContent = Json.decodeValue(httpResponse.body(), returnType);
    }
}

Start at the generated Java-Vert.x client entry point named ApiClient.buildResponseHandler and compare handling for text/plain with the provided /metrics response schema. Reproduce generation with the supplied swagger-codegen command, then verify that a String response is handled without passing non-JSON content to Json.decodeValue.