Add rexml as a default dependency

This gems needs a working XML parser. rexml is no longer a default gem of Ruby,
which means it might not be available. Running a client from a Gemfile
that only contains xmlrpc resulted in load errors.

Author: qnet-herwin

Gemfile

@@ -5,6 +5,5 @@ gemspec
 group :development do
   gem "bundler"
   gem "rake"
-  gem "rexml"
   gem "test-unit"
 end

xmlrpc.gemspec

@@ -18,5 +18,6 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.required_ruby_version = ">= 2.3"
 
+  spec.add_dependency "rexml"
   spec.add_dependency "webrick"
 end