`Browser::start_incognito_context` doesn't start incognito context if browser is started in incognito mode

[d4h0]

Hi,

If a browser is started with the following config:

BrowserConfig::builder().incognito().with_head().build()

...later calls of Browser::start_incognito_context do not open an incognito context.

If .incognito() above is removed, everything works as expected.