Fix native resampler using SwathDefinition as an AreaDefinition

[djhoese]

First fixed by @wroberts4 in #317 but was not cherry-pick-able

If a dataset had a SwathDefinition for an area and also had a 'y' coordinate then the associated if statement was entered when it shouldn't be.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[coveralls]

Coverage remained the same at 69.075% when pulling b56fbb3 on djhoese:bugfix-resample-coords into 0c80f47 on pytroll:master.

[codecov]

Codecov Report

Merging #391 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #391   +/-   ##
=======================================
  Coverage   69.07%   69.07%           
=======================================
  Files         123      123           
  Lines       15722    15722           
=======================================
  Hits        10860    10860           
  Misses       4862     4862

Impacted Files	Coverage Δ
satpy/resample.py	75.75% <100%> (ø)	⬆️

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