Qualifier case is not normalized on parse

[matt-phylum]

The PURL test suite contains a non-canonical PURL pkg:gem/jruby-launcher@1.1.2?Platform=java which is supposed to parse as a PURL with the qualifier platform=java, but packageurl-dotnet parses the qualifier as Platform=java. The qualifier case is normalized only in ToString.

This causes two problems:

1. If somebody writes code that parses a PURL and then tries to obtain the value of a qualifier, they will sometimes not get the expected value because the qualifier case doesn't match.
2. Serializing the PURL and then deserializing the PURL results in a slightly different PURL.