Treat arguments of type Stringable as string

[masi]

<f:argument name="foobar" type="string" />

This will now throw an Exception if used with Stringable. I know that I could make the type string|Stringable, but that is tedious.

Within the context of Fluid I think it is ok to blur the PHP implementation detail and treat them as the same

[s2b]

This should already work, as demonstrated by the attached pull request. Only if you want to be able to access the object that implements Stringable, you can and need to provide the types individually, which prevents early auto-conversion from Stringable to string.

[masi]

Well, yes, it works (now? I had issues, shrug). But the results are different depending in the argument definition and the passed value.

If the definition is "string" then a Stringable object is treated as unsafe and will be HTML escaped.
If it is "string|Stringable" then then no escaping occurs.

[s2b]

Could you try to reproduce that in a test case, similar to my PR?