fix: output original value from array in std.minArray and std.maxArray instead of result of keyF

[eduardosm]

Fixes #790.

[coveralls]

coverage: 55.574% (+0.1%) from 55.46%
when pulling 52bb10f on eduardosm:fix-minArray-maxArray-keyF
into 106c8f0 on google:master.

[johnbartholomew]

Thanks for finding and fixing this!

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[johnbartholomew]

Rebased, added support for the optional onEmpty parameter (for parity with the C++ version which already had this), and merged.

Thanks again for the contribution!