Fix PyBuffer serializer

[hcho3]

#322 silently broke the Protocol Buffer serializer (GetPyBuffer()). This PR fixes the serializer and also adds a testing coverage.

Error when calling GetPyBuffer():

>       return array(a, dtype, copy=False, order=order)
E       RuntimeError: Item size 268 for PEP 3118 buffer format string T{256s=f=f} does not match the dtype V item size 264.

which means that the format string was incorrect. #322 added a new field to ModelParam but did not update the corresponding format string.

[codecov]

Codecov Report

Merging #340 (90db60d) into mainline (2a62c6f) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             mainline     #340   +/-   ##
===========================================
  Coverage       84.93%   84.93%           
  Complexity         42       42           
===========================================
  Files             108      108           
  Lines            8461     8461           
  Branches          472      472           
===========================================
  Hits             7186     7186           
  Misses           1252     1252           
  Partials           23       23           

Impacted Files	Coverage Δ
include/treelite/tree_impl.h	89.43% <100.00%> (ø)

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