[4.x] Apply the default user agent in Http instead of the factory - #94

Summary of Changes

The default user agent Mozilla/5.0 Joomla!/Framework, backported from the CMS in ba2a2b9, was applied in HttpFactory::getHttp() only:

• a client created with new Http() (or one using a transport from getAvailableDriver()) sent no user agent, or cURL's/PHP's, so the behaviour depended on how the client was built;
• the factory wrote the default into the $options it was given. With an ArrayAccess object such as a Registry or ArrayObject, the caller's object was modified in place.

Http::makeTransportRequest() now uses the userAgent option or, if none is set, the default, without writing it back, and the factory no longer touches the options. Visible difference: (new HttpFactory())->getHttp()->getOption('userAgent') is now null unless the option was set; the user agent that is sent is the same.

The finding also suggests a non-browser user agent such as Joomla-Framework-Http/4. That was not changed here, because the current string was chosen deliberately in the backport; it is easy to do separately if wanted.

Testing Instructions

Tests/HttpUserAgentTest.php (new, mocked transport):

• new Http() passes the default user agent to the transport (before: null);
• a configured userAgent is passed;
• HttpFactory::getHttp() leaves an ArrayObject of options unchanged (before: userAgent was added).

Documentation Changes Required

docs/classes/HttpFactory.md: a paragraph on the default user agent and the userAgent option.

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