Bug: View templates using layouts inherit block values

[Abbe98]

If a template using a layout leaves a block undefined the template will use a past block definition from another template already rendered.

The current way to bypass this is to set the block value to {{ "" }} which sets it to an empty string.

[Abbe98]

Dropped this from 0.1.0 as it's not critical and not easily implemented until we implement pre-parsed templates.