caching fingerprint & test update

lib/paperclip/io_adapters/stringio_adapter.rb

@@ -24,10 +24,12 @@ module Paperclip
     end
 
     def fingerprint
-      rewind # start reading from the beginning
-      finger = Digest::MD5.hexdigest(read)
-      rewind
-      finger
+     if (@cached_fingerprint.nil?)
+       rewind # start reading from the beginning
+       @cached_fingerprint = Digest::MD5.hexdigest(read)
+       rewind # for later read()
+     end
+     @cached_fingerprint
     end
 
     def read(length = nil, buffer = nil)

test/io_adapters/stringio_adapter_test.rb

@@ -34,11 +34,15 @@ class StringioFileProxyTest < Test::Unit::TestCase
       assert_equal Digest::MD5.hexdigest(@contents), @subject.fingerprint
     end
 
+    should "generate correct fingerprint after read" do
+      fingerprint = Digest::MD5.hexdigest(@subject.read)
+      assert_equal fingerprint, @subject.fingerprint
+    end
+
     should "generate same fingerprint" do
       assert_equal @subject.fingerprint, @subject.fingerprint
     end
 
-
     should "return the data contained in the StringIO" do
       assert_equal "abc123", @subject.read
     end