Reverse proxy setup "HTTP Web root" not saving.

[cino]

Describe the bug
I'm trying to set up a reverse proxy towards Medusa (domain, path based), which sort of works fine on the proxy end but I can't seem to save the setting called "HTTP Web root". Tried multiple times to enter this field and save it and however it gives a successful message, when I reload the page the input field is empty.

Reason I think I need this field is if I try to access the Medusa interace over my reverse proxy, lets say: example.com/medusa it redirects towards example.com/login/?next=%2Fmedusa . This should redirect towards example.com/medusa/login/

To Reproduce
Steps to reproduce the behavior:

1. Go to General Configuration
2. Click on Interface
3. Scroll down to HTTP Web root
4. Fill the field
5. Save.
6. Reload
7. Tada, empty field

Expected behavior
I expect this field to be stored successfully, which would than solve my issue with my reverse proxy.

Screenshots

Medusa (please complete the following information):

• OS: Linux-3.10.105-armv7l-with-glibc2.4 // DSM 6.2.2-24922 Update 4
• Branch: master
• Commit: ee1b898
• Python version: 2.7.14
• Database version: 44.14

Additional context

I don't have any logs for this, turned on debug logging but couldn't get the interface to show me the debug logs. If there is another location I can find these (via ssh/command line) I'd be glad to try.

Thanks in advance.

[medariox]

Thanks for the bug report. Will be fixed with #7841