tidy: do not send deprecated headers

[flavorjones]

See discussion at #646 which indicates that sending the now-deprecated accept-charset header confuses some (presumably buggy, but real-world) servers.

I'm opening this ticket to drive:

• inventory of the default headers Mechanize is sending
• removal of any deprecated headers, including accept-charset
  • tidy: do not send the deprecated accept-charset header #648

Special thank-you to @MarcoPrins and @tomasz-jasiulek for reporting and investigating this.

[flavorjones]

Inventory

A vanilla request sends the following headers:

• accept-charset => ISO-8859-1,utf-8;q=0.7,*;q=0.7
• accept-encoding => gzip,deflate,identity
• accept-language => en-us,en;q=0.5
• accept => */*
• host
• user-agent

accept-charset

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Charset

Warnings: "Do not use this header. Browsers omit this header and servers should ignore it."

accept-encoding

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Encoding

Warnings: none

accept-language

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language

Warnings: none

accept

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept

Warnings: none

user-agent

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/User-Agent

Warnings: only to servers about browser detection using the user agent being a bad idea

host

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Host

Warnings: none

[flavorjones]

Seems like accept-charset is the only offender. See #648.