Fix is_encrypted for OOXML

[nolze]

No description provided.

[codecov-io]

Codecov Report

Merging #27 into master will decrease coverage by 0.22%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage    36.9%   36.68%   -0.23%     
==========================================
  Files          13       13              
  Lines        1130     1145      +15     
==========================================
+ Hits          417      420       +3     
- Misses        713      725      +12

Impacted Files	Coverage Δ
msoffcrypto/format/ooxml.py	46.87% <42.85%> (-3.7%)	⬇️
msoffcrypto/__init__.py	39.13% <42.85%> (-2.05%)	⬇️

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