Set content length header based on number of bytes, not chars.

Closes vcr#96.

lib/vcr/structs.rb

@@ -188,8 +188,7 @@ def self.from_hash(hash)
     end
 
     def update_content_length_header
-      # TODO: should this be the bytesize?
-      value = body ? body.length.to_s : '0'
+      value = body ? body.bytesize.to_s : '0'
       key = %w[ Content-Length content-length ].find { |k| headers.has_key?(k) }
       headers[key] = [value] if key
     end

spec/vcr/structs_spec.rb

@@ -1,3 +1,5 @@
+# encoding: UTF-8
+
 require 'yaml'
 require 'vcr/structs'
 
@@ -301,6 +303,13 @@ def instance(body)
               inst.update_content_length_header
             }.to change { inst.headers[header] }.from(['3']).to(['0'])
           end
+
+          it 'sets the header according to RFC 2616 based on the number of bytes (not the number of characters)' do
+            inst = instance('aؼ', '2') # the second char is a double byte char
+            expect {
+              inst.update_content_length_header
+            }.to change { inst.headers[header] }.from(['2']).to(['3'])
+          end
         end
       end
     end