Fetch: removed special treatment of forbidden headers.

Fetch: removed special treatment of forbidden headers.

In c43261bad627 (0.7.10), a notion of forbidden headers was introduced
in accordance in Fetch API.  In the API the Forbidden headers are not
allowed to be changed from JavaScript code for security reasons.

The restriction is removed because there are use cases where Host (which
is considered forbidden) is different from the host address in
URL and JavaScript code is expected to be a trusted source (unlike a
browser context).

This closes #638 issue on Github.