Add shorthand for setting JSON encoding as default.

A large number of APIs are JSON only: passing {encoding: 'json'} to
every request is likely to get very repetitive.

This patch adds a .json property to the exported servertest function,
which simply sets the encoding to json by default. This does not
strongarm json encoding though, if another encoding is specified in the
request options, that encoding will be honored.

I suspect users will likely want the following pattern, setting json
format on by default at require-time:

```js
var servertest = require('servertest').json
```

Not sure how to best add this to the readme though.