Ensure required properties are always set

[lkmorlan]

I was creating some vcf files for organizations. I called ::addCompany() but not ::addName() since they do not have first name, last name, etc. The resulting files would not load properly because the N and FN properties are required.

vCard should always output required properties whenever it serializes to a string, even if the properties are empty. Which fields are always output should depend on what version of vCard is being generated.