rubygems/test_case.rb: fix errors in mkmf tests

* lib/rubygems/test_case.rb (Gem#teardown): BASERUBY is not set
  usually.  fix errors in mkmf tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44526 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/test_case.rb

@@ -315,7 +315,11 @@ class Gem::TestCase < MiniTest::Unit::TestCase
   def teardown
     $LOAD_PATH.replace @orig_LOAD_PATH if @orig_LOAD_PATH
 
-    RbConfig::CONFIG['BASERUBY'] = @orig_BASERUBY
+    if @orig_BASERUBY
+      RbConfig::CONFIG['BASERUBY'] = @orig_BASERUBY
+    else
+      RbConfig::CONFIG.delete('BASERUBY')
+    end
     RbConfig::CONFIG['arch'] = @orig_arch
 
     if defined? Gem::RemoteFetcher then