Don't attempt to parse bodies that only contain spaces.

  Rails 2.x head responses return a single space as
  the body. This is due to a Safari bug where the headers
  are not passed if the content length is zero.

lib/twitter/response/parse_json.rb

@@ -7,7 +7,7 @@ class ParseJson < Faraday::Response::Middleware
 
       def parse(body)
         case body
-        when '', nil
+        when /\A^\s*$\z/, nil
           nil
         when 'true'
           true