FEATURE: Add setting to controle the Expect header on S3 calls

Some providers don't implement the Expect: 100-continue support,
which results in a mismatch in the object signature.

With this settings, users can disable the header and use such providers.

lib/s3_helper.rb

@@ -204,6 +204,7 @@ class S3Helper
     }
 
     opts[:endpoint] = SiteSetting.s3_endpoint if SiteSetting.s3_endpoint.present?
+    opts[:http_continue_timeout] = SiteSetting.s3_http_continue_timeout
 
     unless obj.s3_use_iam_profile
       opts[:access_key_id] = obj.s3_access_key_id