Set correct uplevel to warning on supply_default_content_type

[pocke]

This PR fixes uplevel of supply_default_content_type.

Problem

Currently supply_default_content_type specifies an incorrect uplevel to warn method.
For example:

require 'net/http'

url = URI.parse('http://www.example.com/index.html')
req = Net::HTTP::Post.new(url.path)
res = Net::HTTP.start(url.host, url.port) {|http|
  http.request(req)
}
res.body

$ ruby test.rb
/Users/kuwabara.masataka/.rbenv/versions/trunk/lib/ruby/3.2.0+0/net/http/generic_request.rb:186: warning: net/http: Content-Type did not set; using application/x-www-form-urlencoded

The warning points to net/http/generic_request.rb which is not written by the user. The location is not helpful to fix the warning by the user.

Solution

Find the correct uplevel from the caller and specify it.

I also considered using a fixed uplevel for it, like uplevel: 6. But I think caller_location is better than the fixed value.
Because the uplevel is too deep, and supply_default_content_type is called from multiple places so the fixed value doesn't work properly for all callers perhaps.

[mame]

@ko1 said that absolute_path might return nil if a method written in C is called

[nobu]

Maybe:

Suggested change

	uplevel = caller_locations.find_index { |loc| not loc.absolute_path.match?(%r!net/http!) } || 0
	uplevel = caller_locations.find_index { |loc| not loc.absolute_path&.match?(%r!net/http!) } || 0