feat(MongoClient): add appname to list of valid option names

Author: sbilodeau

lib/mongo_client.js

@@ -38,7 +38,7 @@ var validOptionNames = ['poolSize', 'ssl', 'sslValidate', 'sslCA', 'sslCert',
   'serializeFunctions', 'ignoreUndefined', 'raw', 'promoteLongs', 'bufferMaxEntries',
   'readPreference', 'pkFactory', 'promiseLibrary', 'readConcern', 'maxStalenessSeconds',
   'loggerLevel', 'logger', 'promoteValues', 'promoteBuffers', 'promoteLongs',
-  'domainsEnabled', 'keepAliveInitialDelay', 'checkServerIdentity', 'validateOptions'];
+  'domainsEnabled', 'keepAliveInitialDelay', 'checkServerIdentity', 'validateOptions', 'appname'];
 var ignoreOptionNames = ['native_parser'];
 var legacyOptionNames = ['server', 'replset', 'replSet', 'mongos', 'db'];