Bugfix for field with TYPE=CHARACTER.

[kunalbhutani]

Simple bug fix for TYPE=Character in the VCF.

Note that VCF Spec v4.3 lists a TYPE=Character on p.4 https://samtools.github.io/hts-specs/VCFv4.3.pdf

Relevant Text:
1.3 Data types
Data types supported by VCF are: Integer (32-bit, signed), Float (32-bit, formatted to match the regular expression
^[-+]?[0-9]*.?[0-9]+([eE][-+]?[0-9]+)?$, NaN, or +/-Inf), Flag, Character, and String.

Closes #159

[alimanfoo]

Thanks @kunalbhutani. Just to check, the "Character" type means the expected value is a single character, whereas the "String" type means a variable number of characters, is that right? Sounds like it should be obvious but it's not explicitly stated in the spec.

[alimanfoo]

Thanks Kunal, I have manually merged with a slight modification to use 'S1' dtype for Character fields, which should be more efficient than using variable length strings ('object') given we know values should be a single character long. Will be included in 1.1.9 release.