GH-59: Fix ObjectEncoder dropping attributes with falsy values

[ademarco]

Q	A
Type	bug
BC Break	no
Fixed issues	#59

Summary

• Fix truthiness check in ObjectEncoder that silently dropped XML attributes with falsy PHP values (false, 0, 0.0, "")
• Replace $value ? with $value !== null ? so only genuinely absent attributes are skipped
• Add test cases for falsy boolean attribute encoding/decoding

[veewee]

Thanks!