When using the **conditionalHandler** plugin, if a single version route has a `contentType`, but the other routes do not, it will crash as it tries to call `.join` on a undefined `contentType` field.

See for yourself:

``` bug.js
const restify = require('restify');

const server = restify.createServer();

server.get('/', restify.plugins.conditionalHandler([
  {
    contentType: ['application/json'],
    version: '1.5.0', 
    handler: (req, res, next) => {
      res.json({
        version: 0
      })
    }
  },
  {
    // contentType is missing here
    version: '1.1.0', 
    handler: (req, res, next) => {
      res.json({
        version: 1
      })
    }
  }
]));
```