Cannot correctly add a tag containing ampersand ("&")

[NewPProd]

The API does not allow you to upload tags containing ampersands ("&" chars).
The tags are provided as URL parameters, so ampersands are URL encoded to %26.
e.g. I'd like to add a tag for "b&w". URL encoding the "&" gives "%26".

POST https://api.500px.com/v1/photos/12345etc/tags?tags=b%26w

The end result is an incorrect tag visible in the 500px website UI named "b & amp ; w" (without the spaces).

It appears you can use the 500px web UI to correctly enter a tag containing an ampersand.
So how do you use the API to do the same?