Type=Character fields are converted to String

[tomwhite]

Found while working on #198

$ bcftools view --header-only tests/data/vcf/field_type_combos.vcf.gz | grep ICD        
##INFO=<ID=ICD,Type=Character,Number=.,Description="INFO,Type=Character,Number=.">
$ vcf2zarr convert --force tests/data/vcf/field_type_combos.vcf.gz field_type_combos.vcz
$ vcztools view --header-only field_type_combos.vcz | grep ICD
##INFO=<ID=ICD,Number=7,Type=String,Description="INFO,Type=Character,Number=.">