fix overwriting of 'options.type' in 'createJsonClient'

This breaks calling like this:

    this.client = restify.createJsonClient(options);
    this.rawClient = restify.createClient(options);

The `rawClient` would not be the expected HttpClient.
Merge pull request #291 from trentm/fix-client-options

fix overwriting of 'options.type' in 'createJsonClient'