nsq_to_http: make Content-Type configurable

Currently the header is hardcoded to `Content-Type: application/octet-stream`.

While *octet-stream* is a safe default, forcing it quietly is likely to cause confusion for users whose messages are a query string (as in `application/x-www-form-urlencoded`).

Some (most?) web frameworks will not parse/populate the form data as expected (PHP no `$_POST`; werkzeug no `request.form[]`.)

In the interest of backwards compatibility, I'd propose we add a flag to *nsq_to_http*:

`-mime="application/octet-stream"`

If no one has any objections, I'll send a patch.