Export GFF format: . instead of + in the strand (seven) position

https://github.com/The-Sequence-Ontology/Specifications/blob/master/gff3.md

Column 7: "strand"The strand of the feature. + for positive strand (relative to the landmark), - for minus strand, and . for features that are not stranded. In addition, ? can be used for features whose strandedness is relevant, but unknown.

Scenario:

1. Open attached gff file.
2. Strand position (seven) should be "+" instead of ".".