Use 'default' for $asset_tag when none is provided to scripts() or styles()

If, as in your sample code, you simple echo a call to scripts() or styles() on the asset manager, and no tags have been used, nothing will be output. This is because assets are tagged as 'default' when queued and no tag is provided, but NULL is assumed for the tag when a call to scripts() or styles() is made.

This commit should do it:

https://github.com/greenham/Twootstrap/commit/07e8831ca211c0e52afde740db0f3c947f67a142