Parameters with no example value

When a parameter was documented without a example value -- like
`since` in **List All Gists** from [Gist Fox API example](https://github.com/apiaryio/api-blueprint/blob/master/examples/Gist%20Fox%20API.md#list-all-gists-get) -- , aglio generated document gave `since` empty string `""` in example URI like following:

```
GET /gists?since=
```

sometimes, I just want to ignore `since` or other query parameters in URI and let the server set it to its default value, like

```
GET /gists
```

and server set the `since` to 24 hours ago from now.

No source file or test is named in the issue. Start by reproducing the Gist Fox API “List All Gists” example and tracing how an undocumented `since` parameter becomes `since=`; done means the generated example URI can omit that parameter while preserving the server’s default behavior.

Written by the indexing model from the issue text.