made Middleware call ruby 1.9 compatible. We must stop considering th…

…e response body to be UTF-8, it is binary data so #lenght is not the right method to call

lib/pdfkit/middleware.rb

@@ -27,7 +27,7 @@ def call(env)
         headers.delete('ETag')
         headers.delete('Cache-Control')
 
-        headers["Content-Length"] = body.length.to_s
+        headers["Content-Length"] = body.bytes.to_a.size.to_s
         headers["Content-Type"] = "application/pdf"
 
         response = [body]