Fix logging when level is set on the logger, but no level is set on tranports

[jstamerj]

Example code:

    var winston = require('winston');
    var logger = new winston.Logger({
      level: 'warn',
      transports: [new winston.transports.Console()]
    });
    logger.info('should not see this');
    logger.warn('should see this');

Expected output:

    should see this

Actual output:

    should not see this
    should see this

Reason: transport.js sets a default level of 'Info' when no level option is set on a transport. logger.js always uses the transport level (when set) instead of using the level set on the logger. Since a level is always set on every transport, the logger's level is never used.

I changed transport.js so that it does not set a default value for this.level. This fixes the issue.

[indexzero]

This is totally correct, but it is breaking and will go into winston@1.0.0

[indexzero]

Cherry-picked. Thanks! Will go into winston@1.0.0 which will come out shortly.

[FGRibreau]

Now that's better! Thanks @jstamerj @indexzero!