in_http: replace WEBrick::HTTPUtils.parse_query with URI

[daipom]

Which issue(s) this PR fixes:

• Partially Fixes #4648

What this PR does / why we need it: Replace WEBrick::HTTPUtils.parse_query with URI.decode_www_form in in_http.

WEBrick is no longer recommended for production use.

WEBrick::HTTPUtils is just a util, so it might be harmless. However, it would be happy to reduce dependence if possible.

This changes some specifications unintentionally. At least, this makes us unable to use ; delimitter. We need to consider whether this breaking change is acceptable or not.

Docs Changes: Not needed. (There is no impact on the specification.)

Release Note: The same as the title.