[rubygems/rubygems] Allow "default_user_install" to be overridden.

For Ruby re-distributors, automatic user-install might be the right
default. Therefore printing warning about installing into user directory
is not always desirable. Let the default_user_install method be
customizable.

rubygems/rubygems@2320dba544

lib/rubygems/defaults.rb

@@ -235,6 +235,18 @@ def self.default_cert_path
     default_cert_path
   end
 
+  ##
+  # Enables automatic installation into user directory
+
+  def self.default_user_install # :nodoc:
+    if !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
+      Gem.ui.say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
+      return true
+    end
+
+    false
+  end
+
   ##
   # Install extensions into lib as well as into the extension directory.
 

lib/rubygems/installer.rb

@@ -684,10 +684,7 @@ def process_options # :nodoc:
         # * `true`: `--user-install`
         # * `false`: `--no-user-install` and
         # * `nil`: option was not specified
-        if options[:user_install]
-          @gem_home = Gem.user_dir
-        elsif options[:user_install].nil? && !ENV.key?("GEM_HOME") && (File.exist?(Gem.dir) && !File.writable?(Gem.dir))
-          say "Defaulting to user installation because default installation directory (#{Gem.dir}) is not writable."
+        if options[:user_install] || (options[:user_install].nil? && Gem.default_user_install)
           @gem_home = Gem.user_dir
         end
       end