RUBY-2132 disconnect monitoring connection on error (#1939)

Co-authored-by: Oleg Pudeyev <oleg@bsdpower.com>

Author: p-mongo

lib/mongo/server/monitor.rb

@@ -262,6 +262,7 @@ def ismaster
               message = @connection.dispatch_bytes(Monitor::Connection::ISMASTER_BYTES)
               message.documents.first
             rescue Mongo::Error
+              @connection.disconnect!
               @connection = nil
               raise
             end