[Twig] allow stringable objects as attribute values

[kbond]

Q	A
Bug fix?	yes
New feature?	no
Issues	n/a
License	MIT

Allows passing a stringable objects as attribute values.

{# currently #}
<twig:Form :action="stringableUriObject"> {# error because stringableUriObject isn't scalar #}

{# with this PR #}
<twig:Form :action="stringableUriObject"> {# auto converts stringableUriObject to string #}

[seb-jean]

What will this change? :)

[kbond]

What will this change? :)

I've updated the PR description to show what will change.

[smnandre]

Great ! Thanks you :)

Minor if you have time:
The description may be a bit missleading, as your PR has no impact on how you pass arguments to components, but how attributes are rendered.
Maybe illustrate with something in the "component view" instead of the "calling template" ?