channel_as_last_axis decorator fix

[grlee77]

Description

After moving the channel axis to the last position, channel_as_last_axis also needs to update the channel_axis kwarg to -1.

This bug was not revealed by the existing test cases for functions where it was used because these functions seem to only check that channel_axis is not None. See for example:

scikit-image/skimage/exposure/histogram_matching.py

Line 66 in 044ac80

if channel_axis is not None:

The new test here verifies both that the data has been reshaped and the argument set to -1 as expected.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[alexdesiqueira]

Thank you Greg!