ubuntu-device-flash can't specify image server

[Shuduo Sang]

When I use ubuntu-device-flash to generate snappy personal image, the sysmte-image server be automatically specified to http://CN.system-image.ubuntu.com as I'm living in China. But due to some unknown issues from CDN vendor, the file of CN server can't be retrieved successfully. Then I tried to use --server to specify oversea server https://system-image.ubuntu.com but it still use CN server so no help to me.

Reproduce steps:
1, sudo ubuntu-device-flash core rolling --oem=generic-amd64/stable --channel edge -o xenial_core_amd64.img --developer-mode
Determining oem configuration
Starting download of generic-amd64
41.70 KB / 41.70 KB [==============================================================================================================================================] 100.00 % 70.05 KB/s
Done
Starting download of icon for package
38.97 KB / 38.97 KB [=============================================================================================================================================] 100.00 % 115.32 KB/s
Done
Fetching information from server...
Downloading and setting up...
2016/01/11 14:52:56 Get http://CN.system-image.ubuntu.com/pool/ubuntu-99d0fbd5d2290126b0e78de6fbb9e0ecf804881868f035d18c6402211661693c.tar.xz.asc: stopped after 10 redirects

2, sudo ubuntu-device-flash core rolling --oem=generic-amd64/stable --server https://system-image.ubuntu.com --channel edge -o xenial_core_amd64.img --developer-mode
Determining oem configuration
Starting download of generic-amd64
41.70 KB / 41.70 KB [==============================================================================================================================================] 100.00 % 70.84 KB/s
Done
Starting download of icon for package
38.97 KB / 38.97 KB [==============================================================================================================================================] 100.00 % 64.99 KB/s
Done
Fetching information from server...
Downloading and setting up...
2016/01/11 14:53:45 Get http://CN.system-image.ubuntu.com/pool/ubuntu-99d0fbd5d2290126b0e78de6fbb9e0ecf804881868f035d18c6402211661693c.tar.xz.asc: stopped after 10 redirects

Expectation:
retrieve file from server I specified by argument --server

Actual:
still retrieve from CN server

[Shuduo Sang (sangshuduo)]

Interesting thing is --server works good on canonistack to specify different server. Seems the bug only happens in China.