Don't try to show unset display name

In the ReadReceiptMarker title, show the display name only when it is not the
same as the user ID.

src/components/views/rooms/ReadReceiptMarker.js

@@ -185,12 +185,22 @@ module.exports = React.createClass({
 
         let title;
         if (this.props.timestamp) {
-            title = _t(
-                "Seen by %(displayName)s (%(userName)s) at %(dateTime)s",
-                {displayName: this.props.member.rawDisplayName,
-                 userName: this.props.member.userId,
-                 dateTime: formatDate(new Date(this.props.timestamp), this.props.showTwelveHour)},
-            );
+            const dateString = formatDate(new Date(this.props.timestamp), this.props.showTwelveHour);
+            if (this.props.member.userId === this.props.member.rawDisplayName) {
+                title = _t(
+                    "Seen by %(userName)s at %(dateTime)s",
+                    {userName: this.props.member.userId,
+                    dateTime: dateString},
+                );
+            }
+            else {
+                title = _t(
+                    "Seen by %(displayName)s (%(userName)s) at %(dateTime)s",
+                    {displayName: this.props.member.rawDisplayName,
+                    userName: this.props.member.userId,
+                    dateTime: dateString},
+                );
+            }
         }
 
         return (

src/i18n/strings/en_EN.json

@@ -389,6 +389,7 @@
     "Offline": "Offline",
     "Unknown": "Unknown",
     "Replying": "Replying",
+    "Seen by %(userName)s at %(dateTime)s": "Seen by %(userName)s at %(dateTime)s",
     "Seen by %(displayName)s (%(userName)s) at %(dateTime)s": "Seen by %(displayName)s (%(userName)s) at %(dateTime)s",
     "No rooms to show": "No rooms to show",
     "Unnamed room": "Unnamed room",