Fix #6609 and #6587 - Change Content-Length behavior in Rex HTTP

This patches changes two things:

1. If a module has a custom Content-Length, it will respect that
   instead of forcing its own.

2. If a request does not have anything in the body, the
   Content-Length header will not be set.

Fix #6609
Fix #6587

lib/rex/proto/http/client_request.rb

@@ -391,8 +391,9 @@ class ClientRequest
 
   #
   # Return the content length header
+  #
   def set_content_len_header(clen)
-    return "" if opts['chunked_size'] > 0
+    return "" if clen == 0 || opts['chunked_size'] > 0 || (opts['headers'] && opts['headers']['Content-Length'])
     set_formatted_header("Content-Length", clen)
   end
 

spec/lib/rex/proto/http/client_request_spec.rb

@@ -151,7 +151,26 @@ RSpec.describe Rex::Proto::Http::ClientRequest do
       {
         :set_host_header       => { :result => "Host: [2001:DB8::1]:1234\r\n" },
       }
-    ]
+    ],
+
+    [
+      "with modified Content-Length header",
+      default_options.merge({
+        'headers' => { 'Content-Length' => 1337 }
+      }),
+      {
+        :set_content_len_header => { args: 0, result: ''}
+      }
+    ],
+
+    [
+      "with 1024 bytes of Content-Length",
+      default_options,
+      {
+        :set_content_len_header => { args: 1024, result: "Content-Length: 1024\r\n"}
+      }
+    ],
+  
   ].each do |c, opts, expectations|
     context c do
       subject(:client_request) { Rex::Proto::Http::ClientRequest.new(opts) }