Fix freezing of areas before resampling even as strings

[mraspaud]

Dynamic areas passed as strings wouldn't receive the area to freeze on.

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

[djhoese]

Are there any times where max_area would raise an exception, but we have a static AreaDefinition and therefore wouldn't need it?

[mraspaud]

I haven't encountered this so far

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@bc87fed). Click here to learn what that means.
The diff coverage is 60%.

@@            Coverage Diff            @@
##             master     #348   +/-   ##
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  Coverage          ?   69.68%           
=========================================
  Files             ?      123           
  Lines             ?    15877           
  Branches          ?        0           
=========================================
  Hits              ?    11064           
  Misses            ?     4813           
  Partials          ?        0

Impacted Files	Coverage Δ
satpy/resample.py	75.75% <ø> (ø)
satpy/scene.py	84.56% <60%> (ø)

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