Add rdoc as development dependency and fix version-dependent tests

- Add rdoc to gemspec since it was removed from default gems in Ruby 4.0
- Use Gem::Version checks for libxml2 version-dependent test assertions

Author: cfis

libxml-ruby.gemspec

@@ -46,5 +46,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency('logger')
   spec.add_development_dependency('rake-compiler')
   spec.add_development_dependency('minitest')
+  spec.add_development_dependency('rdoc')
   spec.license = 'MIT'
 end