Fix CER/DER encoders to respect open types

[etingof]

Added a bunch of unit tests to CER/DER codecs covering open types.

[codecov-io]

Codecov Report

Merging #167 into master will increase coverage by 0.66%.
The diff coverage is 90.69%.

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   85.61%   86.27%   +0.66%     
==========================================
  Files          29       29              
  Lines        4199     4190       -9     
==========================================
+ Hits         3595     3615      +20     
+ Misses        604      575      -29

Impacted Files	Coverage Δ
pyasn1/codec/ber/encoder.py	89.33% <88.88%> (+0.44%)	⬆️
pyasn1/codec/cer/encoder.py	86.02% <93.75%> (+0.51%)	⬆️
pyasn1/codec/ber/decoder.py	89.51% <0%> (+3.12%)	⬆️

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