Don't try to encode to a nil encoding.

This should allow cassettes recorded on 1.8 to work on 1.9.

Closes #149.

lib/vcr/structs.rb

@@ -32,7 +32,7 @@ def force_encode_string(string, encoding)
           end
 
           def try_encode_string(string, encoding)
-            return string if string.encoding.name == encoding
+            return string if encoding.nil? || string.encoding.name == encoding
             string.encode(encoding)
           rescue EncodingError => e
             struct_type = name.split('::').last.downcase

spec/vcr/structs_spec.rb

@@ -166,6 +166,16 @@ def body_hash(key, value)
           i.response.body.encoding.name.should eq("ISO-8859-1")
         end
 
+        it 'does not attempt to encode the string when there is no encoding given (i.e. if the cassette was recorded on ruby 1.8)' do
+          string = 'foo'
+          string.force_encoding("ISO-8859-1")
+          hash['request']['body']  = { 'string' => string }
+
+          i = HTTPInteraction.from_hash(hash)
+          i.request.body.should eq('foo')
+          i.request.body.encoding.name.should eq("ISO-8859-1")
+        end
+
         context 'when the string cannot be encoded as the original encoding' do
           before do
             Request.stub(:warn)