work around perf regression on 0.7

[KristofferC]

Fixes #100

The 1:length(a) is safe to use since this method is only valid for Array and views thereof.

Before:

julia> @btime colwise!($d,$(Euclidean()), $A, $B);
  638.378 ns (82 allocations: 3.84 KiB)

After

julia> @btime colwise!($d,$(Euclidean()), $A, $B);
  179.346 ns (0 allocations: 0 bytes)

0.6 for reference:

julia> @btime colwise!($d,$(Euclidean()), $A, $B);
  238.585 ns (0 allocations: 0 bytes)

[KristofferC]

Test fails because the tests are run with check-bounds=yes. I added a @test_broken for that but test it when running the file directly, e.g. via include. Test passes locally.