Don't modify encoding of argument to unserialize.

Fixes #20

Author: jqr

lib/php_serialize.rb

@@ -164,7 +164,7 @@ def PHP.unserialize(string, classmap = nil, assoc = false) # {{{
 
 		ret = nil
 		original_encoding = string.encoding
-		string = StringIOReader.new(string.force_encoding('BINARY'))
+		string = StringIOReader.new(string.dup.force_encoding('BINARY'))
 		while string.string[string.pos, 32] =~ /^(\w+)\|/ # session_name|serialized_data
 			ret ||= {}
 			string.pos += $&.size

test/php_serialize_test.rb

@@ -134,4 +134,14 @@ def test_new_struct_creation
       PHP.unserialize(phps)
     end
   end
+
+	def test_encoding_kept
+		s = String.new('s:3:"abc";', encoding: "ISO-8859-1")
+
+		assert_equal "ISO-8859-1", s.encoding.name
+
+		PHP.unserialize(s)
+
+		assert_equal "ISO-8859-1", s.encoding.name
+	end
 end