Don't chunk response body if transform_response_cl is valid

The way as I see, the client will get chunked response from ATS if the origin server issues a chunked response to ATS.

I am wondering whether this can be changed if there is a transfrom plugin exists and the transform can insure transform_response_cl is valid. This can be done by TSVConnWrite(...) with a valid nbytes(not INT64_MAX) in transform handler.

Here is an example, I want to response "abcdefg" in transform handler, no matter what is received from upstream, and I can write code like this in plugin:

static int transform_handler(...)
{
	...
	output.buffer = TSIOBufferCreate();
	output.reader = TSIOBufferReaderAlloc(output.buffer);
	output.vio = TSVConnWrite(output_conn, contp, output.reader, sizeof("abcdefg")-1);

	TSIOBufferWrite(output.buffer, "abcdefg", sizeof("abcdefg")-1);

	TSVIOReenable(output.vio);

	...
}

However, the response body to the client will be chunked if ATS got chunked response from origin server. Maybe we can change this by refining the function HttpTransact::handle_response_keep_alive_headers(...)