Need to emit CRLF between lines in the payload

[jkakar]

If I've understood section 5 of the specification correctly each
line in the payload should be terminated with a CRLF:

        line        = key "=" value CRLF

It looks like this gem is using a newline and only for N-1 lines
emitted:

irb(main):020:0> OpenToken::KeyValueSerializer.serialize("foo" => "bar")
=> "foo=bar"
irb(main):021:0> OpenToken::KeyValueSerializer.serialize("foo" => "bar", "baz" => "quux")
=> "foo=bar\nbaz=quux"