Retain metadata for polyadic assoc-some

[tomdl89]

Fixes issue raised in #91

[tomdl89]

On a related note, reduce-map could benefit from this too: https://github.com/tomdl89/medley/commit/8fd5920491cf15deb55d30baddab69bbf908ee64 I'm not convinced https://github.com/weavejester/medley/issues/13 reached the right conclusion btw. Comparing to map filter remove etc doesn't seem to make sense, as those functions produce a lazy-seq from a potentially different coll, whereas map-vals filter-vals remove-vals etc always take a map and return a map. Indeed, update-vals in latest clojure versions (which is doing the same thing as map-vals) does retain metadata.

[weavejester]

Thanks for the commit, and apologies for the delay in reviewing it. Can you add a simple test just to prevent any regressions in future.

With regard to #13, I was originally basing my decision on mapv, which takes a vector and returns a vector, but discards the metadata. It's interesting that with update-vals they've decided to go the other way.

I'm willing to keep metadata for maps if that's the precedent set by clojure.core. I'm just concerned about keeping behavior consistent.

[NoahTheDuke]

I think the reasoning is that functions that modify an object retain metadata, and sequence functions don't. update-vals follows after assoc and update by retaining metadata, whereas mapv follows after map which does not. I would expect an assoc-like function to retain metadata.

[weavejester]

Then perhaps the rule is that functions with the collection as the first argument retain metadata, and otherwise it doesn't. Hence update-vals retaining metadata, and mapv not doing so.

[weavejester]

Looks like this PR is still waiting on a test. Would you be able to write one @tomdl89, or do you not have time? If needs be, I can write one.