fix(ClientSessions): initialize clientOptions and cluster time

Fixes NODE-1557

Author: Sam Pal

lib/core/sessions.js

@@ -70,6 +70,8 @@ class ClientSession extends EventEmitter {
     }
 
     options = options || {};
+    clientOptions = clientOptions || {};
+
     this.topology = topology;
     this.sessionPool = sessionPool;
     this.hasEnded = false;
@@ -81,12 +83,7 @@ class ClientSession extends EventEmitter {
         typeof options.causalConsistency !== 'undefined' ? options.causalConsistency : true
     };
 
-    options = options || {};
-    if (typeof options.initialClusterTime !== 'undefined') {
-      this.clusterTime = options.initialClusterTime;
-    } else {
-      this.clusterTime = null;
-    }
+    this.clusterTime = options.initialClusterTime;
 
     this.operationTime = null;
     this.explicit = !!options.explicit;