Clean up empty query params before interpolating URI templates

## What

This removes keys with empty values from input objects before interpolating them into templated URIs.

Otherwise an input like this:

```javascript
{
  "foo": 1,
  "bar": ""
}
```

.. Get's mapped to a URI like this:

```
host.com?foo=1&bar=
```

With this change, the case above results in 

```
host.com?foo=1
```

`null` or empty string fields will be ignored. Not sure this is desirable for all cases but it makes sense as a rule of thumb.

![screenshot 2014-05-02 18 51 57](https://cloud.githubusercontent.com/assets/1081/2865931/e127efdc-d222-11e3-9af2-dcce90b687c7.png)


## Why

The same can be achieved by editing the input object manually in the provided text-area and removing the unwanted properties, but that gets annoying when there are too many optional properties.