[rubygems/rubygems] Fix encoding mismatch errors on MinGW

GNU make in MSys is localized to use UTF-8 while Ruby's filesystem
encoding is set to OEM CodePage (e.g., CP932 in Japanese Edition),
the read output from the make has broken encoding and results in
"invalid byte sequence" errors.  As `DESTDIR` is set to a US-ASCII
7bit clean string, matching as binary encoding should have no
problems.

https://github.com/rubygems/rubygems/commit/96a5e7523b

test/rubygems/test_gem_ext_builder.rb

@@ -47,7 +47,7 @@ install:
       Gem::Ext::Builder.make @dest_path, results
     end
 
-    results = results.join "\n"
+    results = results.join("\n").b
 
     assert_match %r{"DESTDIR=#{ENV['DESTDIR']}" clean$},   results
     assert_match %r{"DESTDIR=#{ENV['DESTDIR']}"$},         results
@@ -78,7 +78,7 @@ install:
       Gem::Ext::Builder.make @dest_path, results
     end
 
-    results = results.join "\n"
+    results = results.join("\n").b
 
     assert_match %r{"DESTDIR=#{ENV['DESTDIR']}" clean$},   results
     assert_match %r{"DESTDIR=#{ENV['DESTDIR']}"$},         results