Support default values for non-anonymous structs

[nbedos]

Issue: #274

This adds support for default values on fields of non-anonymous structs. I'm not sure why there was a check for the struct being anonymous prior to calling valueFromTree? I don't see a reason to exclude non-anonymous struct.

[codecov]

Codecov Report

Merging #326 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #326   +/-   ##
======================================
  Coverage    94.6%   94.6%           
======================================
  Files          10      10           
  Lines        2038    2038           
======================================
  Hits         1928    1928           
  Misses         66      66           
  Partials       44      44

Impacted Files	Coverage Δ
marshal.go	92.65% <100%> (ø)	⬆️

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[pelletier]

Thank you for the patch! And sorry for the late reply. I don't recall why that check was there. Tests not breaking is a good enough indication for me that it was not mandatory :)