org.openid4java.util.InternetDateFormat is not thread-safe #205

org.openid4java.util.InternetDateFormat extends SimpleDateFormat which is not 
thread-safe.
An alternative would be to replace the use of InternetDateFormat with 
commons-lang's org.apache.commons.lang3.time.FastDateFormat:
new InternetDateFormat()
->
FastDateFormat.getInstance("yyyy-MM-dd'T'HH:mm:ss'Z'", 
TimeZone.getTimeZone("GMT"), Locale.US)