Fix bug in cachehash that wouldn't update a key-value pair if the key was already in the cache

[phillip-stephens]

Context

While working on adding linters, govet spotted that the assignment to kv here within the Add function was ineffectual.

Since this is very core logic, I split this off into a separate PR so it can get focused attention.

Changes

• Changed Add -> Upsert to be more accurate
• Simplified if logic
• Update value if key is present in cache
• Add unit test for above
• Add function comment