HTTP: added support for multi-valued headers in r.headersOut.

HTTP: added support for multi-valued headers in r.headersOut.

1) Added support for an array of values in assignments:
    r.headersOut['Set-Cookie'] = ['a', '', 'b'] will result in

    Set-Cookie: a
    Set-Cookie: b

    headers in output. All previous Set-Cookie are deleted.

    Only the last element in the table will take effect for standard
    headers such as Content-Type that only accept a single value.

    r.headersOut.foo = [] is the same as

    delete r.headersOut.foo

2) Improved getting of special arrays:
    Set-Cookie is always returned as an array.
    Duplicates of Age, Content-Length, Content-Type, ETag, Expires,
    Last-Modified, Location, Retry-After are ignored.
    All other duplicate header values are joined together with ','.

This closes #266 issue on Github.