[himura467]

Closes #17

• Adds Nack::Response, which builds a response and finishes it into the triplet SPEC.md section 4 requires.
• new(status = 200, headers = [], body = nil) keeps headers as a Nack::Headers and copies body into a binary String that write appends to.
• content_type, content_length and location read and set their fields, and assigning nil deletes them. content_length= raises Nack::SpecError unless given a non-negative Integer.
• redirect(location, status = 302) sets the location and status.
• set_cookie raises ArgumentError on a name, value or attribute outside the rfc6265bis grammar, accepting an expires in 1900..9999 (RFC 5322 section 3.3), and replaces any set-cookie field of the same name (rfc6265bis section 4.1.1). delete_cookie sets an empty cookie that expired at the epoch.
• finish returns [Integer, Array of fields, String], drops the body of a 204, 205 and 304, and wraps the body in an object responding to #each and #trailers (SPEC.md section 4.4) once a trailer is added.