Issue with Content Type

We've identified an issue in the current release of net-http (0.3.2).

When setting a 'Content-Type' header on a POST request like so:
request['Content-Type'] = 'application/json'

It does set the header, however, it does not recognize that the Content-Type is set correctly and adds an additional header to the request with the default application/x-www-form-urlencoded

This can be seen when calling: request.to_hash.inspect:

{"accept-encoding"=>["gzip;q=1.0,deflate;q=0.6,identity;q=0.3"], "Accept"=>["*/*"], "User-Agent"=>["Ruby"], "Host"=>["api.songstats.com"], "Content-Type"=>["application/json"], "connection"=>["close"], "host"=>["api.songstats.com"], "content-length"=>["290"], "content-type"=>["application/x-www-form-urlencoded"]}

The only way to force the application/json header to go through is to explicitly set it in the request:

request.content_type = 'application/json'

So actually it needs to be set twice in order to fully work:

request['Content-Type'] = 'application/json'
request.content_type = 'application/json'

Also when setting request['content-type'] = 'application/json' in lower case it's throwing the error:

NoMethodError: undefined method `split' for nil:NilClass
from /Users/Oskar/.rbenv/versions/3.2.1/lib/ruby/3.2.0/net/http/header.rb:713:in `main_type'

It would be great if this can be streamlined so that setting the content-type in the headers immediately propagates across the entire request, without the need to set it multiple times or be cautious of case sensitivity. This took us a couple of hours to debug because we were un-aware that it currently sends the same header twice in the same request with different values.

Reproduce the POST request and inspect request.to_hash, then read net/http/header.rb around line 713, where the reported lower-case Content-Type failure occurs. Trace how header assignment and content_type interact; done means case-insensitive header assignment produces one correct Content-Type without requiring a second setter.