Parameter-less helper calls don't work inside of attributes

Example: http://ember-twiddle.com/098c793577e88f993e1d

Given a `foo-bar` helper, you aren't able to call it without parameters inside of an attribute: 

```
<a href={{foo-bar}}>look at my href</a>
```

Quick workaround is to specify a parameter that won't be used:

```
<a href={{foo-bar helper=true}}>look at my href</a>
```
