UI, history: update using xhr date header

Makes the 'updated since' contents query based on any previous update's
ajax Date response header instead of the client time. This may offer
more protection against incorrect client time.

This works (and may work better than the previous method) because the
Date header is floored and not rounded and seems to mark a time before
the contents query is made.

Resolves #2242 (hopefully)

client/galaxy/scripts/mvc/history/history-model.js

@@ -172,10 +172,16 @@ var History = Backbone.Model
         var self = this;
 
         var lastUpdateTime = self.lastUpdateTime;
-        self.lastUpdateTime = new Date();
-        // note if there was no previous update time, all summary contents will be fetched
+        // note: if there was no previous update time, all summary contents will be fetched
         return self.contents.fetchUpdated( lastUpdateTime )
-            .done( _.bind( self.checkForUpdates, self ) );
+            .done( function( response, status, xhr ){
+                var serverResponseDatetime;
+                try {
+                    serverResponseDatetime = new Date( xhr.getResponseHeader( 'Date' ) );
+                } catch( err ){}
+                self.lastUpdateTime = serverResponseDatetime || new Date();
+                self.checkForUpdates( options );
+            });
     },
 
     /**  */
@@ -208,7 +214,6 @@ var History = Backbone.Model
                     } else {
                         // otherwise, let listeners know that all updates have stopped
                         self.trigger( 'ready' );
-                        // self.lastUpdateTime = null;
                     }
                 });
         }