[rubygems/rubygems] Improve gemspec assignment and error message

https://github.com/rubygems/rubygems/commit/dc70c5a192

lib/rubygems/commands/build_command.rb

@@ -62,10 +62,8 @@ Gems can be saved to a specified filename with the output option:
 
   private
 
-  def build_gem(gemspec = get_one_optional_argument)
-    unless File.exist?(gemspec)
-      gemspec += ".gemspec" if File.exist?(gemspec + ".gemspec")
-    end
+  def build_gem(gem_name = get_one_optional_argument)
+    gemspec = File.exist?(gem_name) ? gem_name : "#{gem_name}.gemspec"
 
     if File.exist?(gemspec)
       spec = Gem::Specification.load(gemspec)

test/rubygems/test_gem_commands_build_command.rb

@@ -189,7 +189,7 @@ class TestGemCommandsBuildCommand < Gem::TestCase
     end
 
     assert_equal '', @ui.output
-    assert_equal "ERROR:  Gemspec file not found: some_gem\n", @ui.error
+    assert_equal "ERROR:  Gemspec file not found: some_gem.gemspec\n", @ui.error
   end
 
   def test_execute_outside_dir