Fix issue with getting the wrong preferred values for the scale bar.

[ericpre]

Fixes the following issue:

import matplotlib.pyplot as plt
import numpy as np
from matplotlib_scalebar.scalebar import ScaleBar
plt.figure()
image = np.arange(25).reshape((5, 5))
plt.imshow(image)
scalebar = ScaleBar(1, units='px', dimension='pixel-length')
plt.gca().add_artist(scalebar)

While we would expect:

It seems that the wrong preferred value is picked up when looking up the _PREFERRED_VALUES list.

[codecov-io]

Codecov Report

Merging #23 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   52.07%   52.23%   +0.15%     
==========================================
  Files           4        4              
  Lines         603      605       +2     
==========================================
+ Hits          314      316       +2     
  Misses        289      289

Impacted Files	Coverage Δ
matplotlib_scalebar/scalebar.py	91.69% <100%> (+0.06%)	⬆️

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[ppinard]

Good catch! Thank you for the fix