Cast srcset to string before trimming explode

[EmanuelOster]

When using the following example, Fluid interprets the srcset as int, which breaks the trimExplode function.

<sms:image
    image="{image.0}"
    breakpoints="{
    0: {
          'cropVariant': 'some_variant',
          'media': '(min-resolution: 2x)',
          'srcset': '600',
          'sizes': '300px, 100vw',
          },
[...]

This PR resolves this by casting the value to string before handing it over to the function