[Feature] Use custom user agent about web-cache-vulnerability-scanner

[m10x]

Hello @ItsIgnacioPortal
The Readme had an error which is now fixed. It is already possible to set a custom user-agent. The --setheaders or short -sh flag lets you specify custom values for arbitrary headers (except Cookies and Content-Type mainly because Golang handles them a bit different). To set a custom user-agent, use for example:
-sh "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) ...
or
-sh "User-Agent: foobar

If you want to set more than 1 header, you have to link to a file which contains them. Therefore you can use the header_list template. This template already contains as an example how to change the user-agent.
To use the template add -sh "file:/path/to/header_list" as flag.

If there's still anything unclear or if you have any other requests, feel free to reply or open another issue.