public Optional<String> determineRequestContentType(ToShapeId operation, String documentContentType) {
        return determineRequestContentType(operation, documentContentType, null);
    }
Returns the expected request Content-Type of the given operation.

<p>See {@link #determineRequestContentType(ToShapeId, String, String)}
for documentation on how the content-type is resolved.

@param operation Operation to determine the content-type of.
@param documentContentType Content-Type to use for protocol documents.
@return Returns the optionally resolved content-type of the request.
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/HttpBindingIndex.java