[DOC] Enhanced RDoc for Net::HTTPHeader

[BurdetteLamar]

PR to emphasize that setters are mostly about requests, while getters are mostly about responses:

• Moved setter sections before getter sections; you do set/request before response/get.
• Added introductory remark to setters section.
• Added #add_field to setters section.
• Added introductory remark to getters section.
• Added #fetch to getters section.
• Fine-tuned distinction between string field value and array field value (a few places).
• Changed examples for getters to use a response object instead of a request object.