fix(SDAM): emit SDAM events on close and reconnect

* Emit SDAM events on close and reconnect:

According to the SDAM specification, when a server isMaster fails either
due to a network issue, the server is not available, or ok !== 1 then
the server type must change to Unknown. This must then emit a
serverDescriptionChanged event since the type has changed.

This pull emits that event now when the server is closed to set it as
Unknown, and when the server reconnects to set it back to the proper
type. This was previously happening on single server types.

* Fix typo in comment

lib/topologies/server.js

@@ -312,6 +312,13 @@ var eventHandler = function(self, event) {
         delete servers[self.id];
       }
 
+      if (event === 'close') {
+        // Closing emits a server description changed event going to unknown.
+        sdam.emitServerDescriptionChanged(self, {
+          address: self.name, arbiters: [], hosts: [], passives: [], type: 'Unknown'
+        });
+      }
+
       // Reconnect failed return error
       if(event == 'reconnectFailed') {
         self.emit('reconnectFailed', err);
@@ -331,6 +338,11 @@ var eventHandler = function(self, event) {
 
       // Reconnect event, emit the server
       if(event == 'reconnect') {
+        // Reconnecting emits a server description changed event going from unknown to the
+        // current server type.
+        sdam.emitServerDescriptionChanged(self, {
+          address: self.name, arbiters: [], hosts: [], passives: [], type: sdam.getTopologyType(self)
+        });
         return self.emit(event, self);
       }