fix tests

[jwahlstrand]

The ColorTypes function gray doesn't accept integers greater than 1 now, so handle integer matrices differently, maintaining the old behavior of clamping values greater than 1.

Also fix another broken test where promote_type returns Any instead of Union{}.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.62%. Comparing base (acaab89) to head (0cbe779).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ImageView.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
- Coverage   84.75%   82.62%   -2.14%     
==========================================
  Files           6        9       +3     
  Lines         866      892      +26     
==========================================
+ Hits          734      737       +3     
- Misses        132      155      +23     

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