Full support for meta tags that allow multiple values

Marked #2664062: 'multiple' option not working correctly, e.g. og:see_also as duplicate.

I proposed #2589203: Explore moving to one field item per metatag instead of a serailized blob for this, but it was voted down

I'm trying to use og_locale_alternative but not able to get it working for multiple values, it simply shows one with all values, any idea if this is fixed already in dev version?

I checked dev version and don't see any code for the support, adding patch to allow support for property based meta tags.

Corrected output.

Hi Damien, seems for the change in MetatagManager related in failure; either we update the code to avoid suffixing with index for tags that are not multiple or update test case, I'm not able to confirm if other failures are because of this change, but I'll work on them tonight and keep the post updated. Please update here if you find some useful info.

Refactoring to get it passing in tests.

This patch cannot be applied against the latest dev version (8.x-1.x-dev)

I've tried using the latest patch from #17, but this is just giving me empty tags for everything - just the opening & closing bracket.

I am testing with 8.x-1.2 on Drupal 8.3.5, but I've checked and the only changes between that & dev are modifications to tests - nothing that should affect this.

I'm stepping through things now to see if I can work out the cause.

I think there was too much nesting of arrays returned by MetatagManager::generateRawElements().

I've tweaked the patch from #17 slightly to reduce a level of nesting & everything appears to work as it should.

Updating status.

The test failures were because MetaItempropBase needed to handle multiple values too. Patch fixes that.

I also noticed that MetaItempropBase didn't check for replacing secure URLs, so I've added that in too (though maybe that should be a different patch?).

I'm not sure if this is enough though - do all of the base classes need this? And then potentially all of the classes that extend & overwrite this?

I think if the changes are applied to the class MetaNameBase directly, any of the extended classes will handle multiple values.

New patch

Patch #25 not work for me. Metatag 8.x-1.x-dev

#28 introduced some code duplication over different base classes. Not sure why this was necessary. The 7.x Metatag makes no such distinction, so I moved this code back to MetaNameBase.

Attached patch introduces the following changes:
- Moved support for multiple values back to MetaNameBase
- Added test coverage for an og:image:url metatag with multiple values
- Rerolled against 8.x-1.x

the patch from #32 works well for me