Default headers in http driver

[bundleman]

To Reproduce
The http driver adds headers by default, if the user wants to set his own values for these headers, then these values are duplicated. For example, by default, the Accept-Language header is set with the value en-US, en; q = 0.9, ru; q = 0.8, the user wants to set his own value sq; q = 0.6, but the previous value is not replaced by the custom one, but is added to the value by default - en-US, en; q = 0.9, ru; q = 0.8 sq; q = 0.6.

Expected behavior
User headers should replace the default headers

Additional context
Reproducible on all ferret versions