Provide a default host header to make request as valid as possible

While working on this sinatra/sinatra#2053 in
our project. I noticed than when using Webmock, sinatra logs and
especially the enforced rack-protection were showing this kind of logs:

```
D, [2024-11-22T13:05:16.798156 #26673] DEBUG -- : Rack::Protection::HostAuthorization @all_permitted_hosts=[".company.com"] @permitted_hosts=["company.com"] @domain_hosts=[/\A(?-mix:[a-z0-9\-.]+)company\.com\z/i] @ip_hosts=[] origin_host="" forwarded_host=nil
```

As you can see, `origin_host` is empty, because the header is missing.

When not using webmock, we fallback on `net/http` host header setup.
https://github.com/ruby/net-http/blob/cfbbb50c931a78fc2b5c731b9abeda161e1dfdd1/lib/net/http.rb#L2482

Author: benoittgt

lib/webmock/http_lib_adapters/net_http.rb

@@ -257,6 +257,10 @@ def self.request_signature_from_request(net_http, request, body = nil)
 
       headers = Hash[*request.to_hash.map {|k,v| [k, v]}.inject([]) {|r,x| r + x}]
 
+      # Prevent empty Host header by providing similar behavior to Net::HTTP
+      # https://github.com/ruby/net-http/blob/cfbbb50c931a78fc2b5c731b9abeda161e1dfdd1/lib/net/http.rb#L2482
+      headers['host'] = URI(uri).host if headers['host'].nil?
+
       if request.body_stream
         body = request.body_stream.read
         request.body_stream = nil