Update HttpHeaderDateFormat.java

Output for "E, dd-MMM-y HH:mm:ss z"
on java 1.6 I get: Wed, 22-May-13 09:11:41 GM
on java 1.7 I get: Wed, 22-May-2013 09:11:14 GMT

To be uniform on both jdks.. modified it to
"E, dd-MMM-yy HH:mm:ss z"

learnt from - http://stackoverflow.com/questions/16687298/simpledateformat-different-behaviour-depending-on-java-version