allow mimemagic objects to be used as hash keys

lib/mimemagic.rb

@@ -91,10 +91,16 @@ class MimeMagic
   end
 
   # Allow comparison with string
-  def ==(x)
-    type == x.to_s
+  def eql?(other)
+    type == other.to_s
   end
 
+  def hash
+    type.hash
+  end
+
+  alias == eql?
+
   private
 
   def self.child?(child, parent)