Collection filters should check truthy-ness when no value set.

**- Do you want to request a *feature* or report a *bug*?**
bug

**- What is the current behavior?**
Currently, if a filter is set on a collection, without setting a value:
```yaml
filter: {field: "contact"}
```
The CMS will only show files which do *not* contain this field.

**- If the current behavior is a bug, please provide the steps to reproduce.**

**- What is the expected behavior?**
In my opinion, this is counter-intuitive, and instead should check whether the field *does* exist. If no value is set, the CMS should show any file which contains the field, regardless of the value. For required fields, they always exist so this wouldn't matter, but for optional fields it does.

If people wanted to use the old behavior (checking for non-existent fields), I believe they would be able to set `value: undefined`.

**- Please mention your CMS, node.js, and operating system version.**

v1.0.3+master

**- Please link or paste your `config.yml` below if applicable.**