force scalar context when not in the lvalue case.

Funny - I actually ended up solving a very similar problem in https://metacpan.org/module/Hash::MostUtils#hash_slice_by-OBJECT-LIST
However, whereas mslice() handles lvalue assignments correctly, hash_slice_by() handles call context in the non-lvalue case correctly.

This pull request fixes mslice()'s non-lvalue case to explicitly use scalar context.

The original commit note follows:

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drive-by: fix a deprecation warning on this line in t/01-mslice.t:

   for my $meth qw/x y/ { ... }