Add Ruby mask implementation

lib/websocket/mask.rb

@@ -0,0 +1,14 @@
+module WebSocket
+  module Mask
+    def self.mask(payload, mask)
+      return payload if mask.nil? || payload.empty?
+
+      payload.tap do |result|
+        payload.bytesize.times do |i|
+          result.setbyte(i, payload.getbyte(i) ^ mask.getbyte(i % 4))
+        end
+      end
+    end
+
+  end
+end