Open files in binary modes for copying in MSpec's #cp

* See https://bugs.ruby-lang.org/issues/13570.
* Found by MSP-Greg (Greg L).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58991 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/mspec/lib/mspec/helpers/fs.rb

@@ -1,8 +1,8 @@
 class Object
   # Copies a file
   def cp(source, dest)
-    File.open(dest, "w") do |d|
-      File.open(source, "r") do |s|
+    File.open(dest, "wb") do |d|
+      File.open(source, "rb") do |s|
         while data = s.read(1024)
           d.write data
         end