Fix map filter with slice of structs

[ofavre]

The previous map implementation worked only for slices of maps. By using the library's values package this implementation should work with all compatible types.

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes. (at least on modified files)
• [x] New and changed code is covered by tests.
• [ ] Performance improvements include benchmarks.
• [x] Changes match the documented (not just the implemented) behavior of Shopify.