ZnHeaders should allow custom or empty Server info

Hi Sven,

when running a Zinc Server

    ZnServer startDefaultOn: 8080.

the server currently reveals the framework, the framework version and the Pharo version
the server is running on as part of the HTTP communication.

For instance

  (ZnEasy get: '<a href="http://localhost:8080'">http://localhost:8080') headers  at: 'Server'

would give

  Zinc HTTP Components 1.0 (Pharo/7.0)

in the Server Header.

While this is nice for debugging and during development this can be a problem in more
secured/pentested environments where usually specific headers are turned off to not
give specific informations to an attacker (to allow him to find vulnerabilities of specific
versions or servers).

While headers could be changed rewritten in front-ends or load balancing servers we
can not assume that Pharo images work as backend server always. Webservers like Nginx,
Apache, IIS, Express in JS, PHP and other allow to disable/modify these headers.

Using the attached changeset it is possible to modify the Server header and customize it:

 ZnHeaders serverString: 'MoreSecureServer'

If the server string is not customized the default server string is used as before.

We should make sure this is integrated into Pharo 7 but also into the Zinc framework.
I opened a bug https://pharo.fogbugz.com/f/cases/22317/ZnHeaders-should-allow-custom-or-empty-Server-info

As Zinc is also managed externally how should we proceed?

Thx
T.