.toString() coerces null to string:null

Simple test case:
```js
'use strict';

const Query = require('rql/query').Query;
const testPredicate = new Query().eq('foo', null);
console.log(testPredicate.toString());
```

Expected output:
`eq(foo,null)`

Actual output:
`eq(foo,string:null)`

Going to fix this in `greatcare/rql`, so that null values no longer get coerced. This is probably due to the special-case treatment of null in the first lines of query's `encodeValue`.