How would I pass and entry's field value to an include? I have this:

{% include "partials/includes/page-header" with { 'title' : '{{ entry.heading }}' } %}


But it seems that nothing I do will actually pass {{ entry.heading }} as 'title' in the include.
You'll never need to put Twig tags within other Twig tags... Try it like this:

{% include "partials/includes/page-header" with { 'title' : entry.heading } %}