Make --default and --install-dir play nice together

With this combination of flags, `rubygems` should install the default
specification to `--install-dir`, not to the default rubygems
installation.

I doubt this has any realworld use cases, but fixing it is handy for me
in order to be able to write a test for a bug fix in `bundler` related
to `bundle clean --force` removing executables of default gems.

lib/rubygems/installer.rb

@@ -433,7 +433,7 @@ def spec_file
   #
 
   def default_spec_file
-    File.join Gem.default_specifications_dir, "#{spec.full_name}.gemspec"
+    File.join gem_home, "specifications", "default", "#{spec.full_name}.gemspec"
   end
 
   ##

test/rubygems/test_gem_installer.rb

@@ -2170,6 +2170,23 @@ def test_default_gem_with_exe_as_bindir
     assert_equal ['exe/executable'], default_spec.files
   end
 
+  def test_default_gem_to_specific_install_dir
+    @gem = setup_base_gem
+    installer = util_installer @spec, "#{@gemhome}2"
+    installer.options[:install_as_default] = true
+
+    use_ui @ui do
+      installer.install
+    end
+
+    assert_directory_exists File.join("#{@gemhome}2", 'specifications')
+    assert_directory_exists File.join("#{@gemhome}2", 'specifications', 'default')
+
+    default_spec = eval File.read File.join("#{@gemhome}2", 'specifications', 'default', 'a-2.gemspec')
+    assert_equal Gem::Version.new("2"), default_spec.version
+    assert_equal ['bin/executable'], default_spec.files
+  end
+
   def test_package_attribute
     gem = quick_gem 'c' do |spec|
       util_make_exec spec, '#!/usr/bin/ruby', 'exe'