Fix Issue 12434 - std.algorithm.sum of immutable array too

https://d.puremagic.com/issues/show_bug.cgi?id=12434

Simply uses the inference's `Unqual` type as a seed, rather than the straight up result.

Also add some unittesting while at it.
Merge pull request #2071 from monarchdodra/12434

Fix Issue 12434 - std.algorithm.sum of immutable array too