Why can't the user agent be changed?

[josesa]

In the code, the AddHeader function explicitly checks for the User-Agent entry and ignores it.

I could not find in the documentation (or code) any reasoning behind this.

[ktr0731]

There is no specific reason.
However, due to an implementation problem with the Go library, the user-agent needs to be specified with a special option, unlike other headers. So, the current implementation doesn't allow specifying the user-agent, but it's possible to support the user-agent modification.