Add defaults to identifiers in expressions when using {% include ... with ... %}

Previously, using {% include ... with ... %} would add defaults to
identifiers in variable tags ({{ ... }}), but not to identifiers in
expression tags ({% ... %}).

This pull request makes sure that defaults are added in expressions as well.

Example:
  Assume that foo.html contains the following:
    foo is {{foo}}
    foo is {% ifequal foo "bar" %}bar{% else %}not bar{% endifequal %}

  Previously, {% include foo.html with foo="bar" %} would produce:
    foo is bar
    foo is not bar

  Now, it instead produces:
    foo is bar
    foo is bar