fix: support write concern provided as string in fromOptions

Author: mbroadst

lib/write_concern.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const kWriteConcernKeys = new Set(['w', 'wtimeout', 'j', 'fsync']);
+
 /**
  * The **WriteConcern** class is a class that represents a MongoDB WriteConcern.
  * @class
@@ -51,6 +53,14 @@ class WriteConcern {
     }
 
     if (options.writeConcern) {
+      if (typeof options.writeConcern === 'string') {
+        return new WriteConcern(options.writeConcern);
+      }
+
+      if (!Object.keys(options.writeConcern).some(key => kWriteConcernKeys.has(key))) {
+        return;
+      }
+
       return new WriteConcern(
         options.writeConcern.w,
         options.writeConcern.wtimeout,