Fix mersi2_l1b reader setting sensor as a set object

[djhoese]

The DataArray 'sensor' attribute should be a single string right now. Rayleigh correction was complaining about not being able to serialize the 'sensor' attribute.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #761 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #761   +/-   ##
=======================================
  Coverage   81.61%   81.61%           
=======================================
  Files         157      157           
  Lines       22636    22636           
=======================================
  Hits        18475    18475           
  Misses       4161     4161

Impacted Files	Coverage Δ
satpy/readers/mersi2_l1b.py	96.38% <100%> (ø)	⬆️

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[coveralls]

Coverage remained the same at 81.618% when pulling c3c717f on djhoese:bugfix-mersi2-sensor-name into 1bfbac6 on pytroll:master.