add support for dashed attributes (e.g. data-role)

I am using radius to render rails templates for a site that uses a lot of data-* attributes in tags.
Attempting to specify such an attribute, e.g.
<r:tag data-value="43"></r:tag>
raises an exception as radius does not expect the attribute names to contain dashes.

The fix is quite simple. Tested on MRI 1.9.3 and 2.0.0.