I do not believe that httpNow() is going to return zulu time you expect based on the format of the date you used in your question.
Following are a couple of different approaches using various date formats and techniques. Only the last approach provides a Zulu date/time.
There are likely other ways to do this:
1) Using a JSR-223 step and sending in httpNow provided this resulting date/time:
String s = "{{=httpNow()}}";
String dt = testExec.parseInState( s );
testExec.setStateValue("fl_date", dt );
// value of fl_date = FRI, 29 Jul 2016 08:05:29 CDT
2) Using the OOTB date time Filter found under Filters / Utility option:
// Use Filter Date Pattern = yyyy-MM-dd'T'HH:mm:ss'Z'
// Converted value = 2016-07-29T08:05:29Z
// Use Filter Date Pattern = yyyy-MM-dd'T'HH:mm:ssZ
// Use Filter Date Pattern = 2016-07-29T08:05:29-0500
3) Using JSR-223 and SimpleDate format to get a Zulu representation
// I suppose you could run httpNow() through this logic so long as you change the SimpleDateFormat to accept accordingly
import java.util.Date;
import java.text.SimpleDateFormat;
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'");
sdf.setTimeZone(TimeZone.getTimeZone("GMT"));
Date now = new Date();
String zuluDate = sdf.format(now);
testExec.setStateValue("fl_zuluDate", zuluDate);
// value of fl_zuluDate is: 2016-07-29T13:05:29Z