Fix sha256 auth content header fetch for Rack driver

Author: taylorthurlow

lib/api_auth/request_drivers/rack.rb

@@ -53,7 +53,12 @@ def content_type
       end
 
       def content_hash
-        find_header(%w[X-AUTHORIZATION-CONTENT-SHA256])
+        find_header(%w[
+          HTTP_X_AUTHORIZATION_CONTENT_SHA256
+          X_AUTHORIZATION_CONTENT_SHA256
+          HTTP-X-AUTHORIZATION-CONTENT-SHA256
+          X-AUTHORIZATION-CONTENT-SHA256
+        ])
       end
 
       def original_uri