Support to respect default values from attr class

[shwetas1205]

These changes are to fix #10 or #14

I added one unit test to support creating the attr class without providing optional value.

@Tinche let me know your feedback on the same.

[codecov]

Codecov Report

Merging #15 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   99.03%   99.05%   +0.01%     
==========================================
  Files          18       18              
  Lines         935      948      +13     
==========================================
+ Hits          926      939      +13     
  Misses          9        9

Impacted Files	Coverage Δ
cattr/converters.py	100% <100%> (ø)	⬆️
tests/metadata/test_roundtrips.py	100% <100%> (ø)	⬆️
tests/test_structure_attrs.py	100% <100%> (ø)	⬆️

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[Tinche]

Hi,

I'm sick ATM so it might be a few days until I get around to this. Thanks :)

[Tinche]

Hello,

thanks for this. I'll take it in and maybe refactor it a little. Cheers!