Ensure plugin_generator adds to new line in Gemfile

Ensure plugin_generator adds to new line in Gemfile, even if the Gemfile
does not end with an empty line.

[Lisa Ugray, Nikita Penzin]

railties/lib/rails/generators/rails/plugin/plugin_generator.rb

@@ -167,7 +167,7 @@ def gemfile_entry
 
       gemfile_in_app_path = File.join(rails_app_path, "Gemfile")
       if File.exist? gemfile_in_app_path
-        entry = "gem '#{name}', path: '#{relative_path}'"
+        entry = "\ngem '#{name}', path: '#{relative_path}'"
         append_file gemfile_in_app_path, entry
       end
     end

railties/test/generators/plugin_generator_test.rb

@@ -530,10 +530,11 @@ def test_creating_plugin_in_app_directory_adds_gemfile_entry
     gemfile_path = "#{Rails.root}/Gemfile"
     Object.const_set("APP_PATH", Rails.root)
     FileUtils.touch gemfile_path
+    File.write(gemfile_path, "#foo")
 
     run_generator
 
-    assert_file gemfile_path, /gem 'bukkits', path: 'tmp\/bukkits'/
+    assert_file gemfile_path, /^gem 'bukkits', path: 'tmp\/bukkits'/
   ensure
     Object.send(:remove_const, "APP_PATH")
     FileUtils.rm gemfile_path