Add file modes. Replace magic values with constants.

lib/fakefs/file.rb

@@ -2,6 +2,15 @@ module FakeFS
   class File
     PATH_SEPARATOR = '/'
 
+    MODES = [
+      READ_ONLY           = "r",
+      READ_WRITE          = "r+",
+      WRITE_ONLY          = "w",
+      READ_WRITE_TRUNCATE = "w+",
+      APPEND_WRITE_ONLY   = "a",
+      APPEND_READ_WRITE   = "a+"
+    ]
+
     def self.extname(path)
       RealFile.extname(path)
     end
@@ -69,7 +78,7 @@ module FakeFS
       FileSystem.find(symlink.target).to_s
     end
 
-    def self.open(path, mode='r', perm = 0644)
+    def self.open(path, mode=READ_ONLY, perm = 0644)
       if block_given?
         yield new(path, mode, perm)
       else

lib/fakefs/file_system.rb

@@ -54,7 +54,7 @@ module FakeFS
       files.each do |f|
         if RealFile.file?(f)
           FileUtils.mkdir_p(File.dirname(f))
-          File.open(f, 'w') do |g|
+          File.open(f, File::WRITE_ONLY) do |g|
             g.print RealFile.open(f){|h| h.read }
           end
         elsif RealFile.directory?(f)