Merge pull request sferik#309 from twoism/support_for_head_responses

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

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