Clear record timestamp after save

Since changeset f63036a91c9f, the timestamp is sent also on button calls but if
the record is saved just before this generate an concurrency exception. Indeed
the _timestamp must be cleared just after the save (because the record is not
reloaded before the button call).

issue4644
review9151002

src/model.js

@@ -397,6 +397,7 @@
                 }
             }
             prm.done(function() {
+                this.cancel();
                 this.reload();
             }.bind(this));
             // TODO group written
@@ -920,6 +921,7 @@
         cancel: function() {
             this._loaded = {};
             this._changed = {};
+            this._timestamp = null;
         },
         get_loaded: function(fields) {
             if (!jQuery.isEmptyObject(fields)) {