retrofit/src/main/java/retrofit2/Retrofit.java
public <T> Converter<T, RequestBody> requestBodyConverter(Type type,
      Annotation[] parameterAnnotations, Annotation[] methodAnnotations) {
    return nextRequestBodyConverter(null, type, parameterAnnotations, methodAnnotations);
  }
public <T> Converter<T, RequestBody> requestBodyConverter(Type type,
      Annotation[] parameterAnnotations, Annotation[] methodAnnotations) {
    return nextRequestBodyConverter(null, type, parameterAnnotations, methodAnnotations);
  }
Returns a {@link Converter} for {@code type} to {@link RequestBody} from the available
{@linkplain #converterFactories() factories}.

@throws IllegalArgumentException if no converter available for {@code type}.