Tag / value enumeration values for Reference Category a "-" should be converted to "_"

[goneall]

Example - reference category PACKAGE-MANAGER should be converted to the enum PACKAGE_MANAGER when deserializing and converted back to the dash when serializing.

[goneall]

In reviewing the spec, only Reference Category uses dashes.

[goneall]

Based on spdx/spdx-spec#473 comments, closing this issue as the decision in the SPDX Spec issue is to leave the serialization as is.

[goneall]

Note that I made changes in a branch named issue16 if we change our minds and would like to convert underscrores to dashes in the serialization.