Fix restrict on an AxisArray with non-1 axes

[timholy]

This may seem like a bit of a rareified combination, but if you're running ImageFiltering 0.5.3+ and OffsetArrays 0.10.0+, you can trigger it on Images master with

using Images, TestImages
img = centered(testimage("mri"))
restrict(img)

I'm a bit unhappy about the implementation of ofaxes (it basically only supports Base and OffsetArrays), but I don't have a better idea ATM.

[codecov]

Codecov Report

Merging #779 into master will increase coverage by <.01%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #779      +/-   ##
==========================================
+ Coverage   58.81%   58.81%   +<.01%     
==========================================
  Files          10       10              
  Lines        1231     1236       +5     
==========================================
+ Hits          724      727       +3     
- Misses        507      509       +2

Impacted Files	Coverage Δ
src/algorithms.jl	61.27% <81.81%> (-0.02%)	⬇️

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[codecov]

Codecov Report

Merging #779 into master will increase coverage by <.01%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #779      +/-   ##
==========================================
+ Coverage   58.81%   58.81%   +<.01%     
==========================================
  Files          10       10              
  Lines        1231     1236       +5     
==========================================
+ Hits          724      727       +3     
- Misses        507      509       +2

Impacted Files	Coverage Δ
src/algorithms.jl	61.27% <81.81%> (-0.02%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fc31f2c...7b9041a. Read the comment docs.

[timholy]

If someone has better ideas we can always improve this later, so I'll merge this.