Fix some small issues

[klaasnicolaas]

• Fix error when int value is missing
• Add an default for the use_json parameter

[codecov-commenter]

Codecov Report

Merging #18 (18cbbfe) into master (3742080) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #18   +/-   ##
=======================================
  Coverage   87.09%   87.09%           
=======================================
  Files           4        4           
  Lines          93       93           
  Branches       10       10           
=======================================
  Hits           81       81           
  Misses          9        9           
  Partials        3        3           

Impacted Files	Coverage Δ
omnikinverter/models.py	100.00% <ø> (ø)
omnikinverter/omnikinverter.py	76.47% <ø> (ø)

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