[rubygems/rubygems] Prevent a warning: net/http: Content-Type did not set

`make test-all RUBYOPT=-w` in the ruby/ruby repo printed the warning.
This change prevents the warning by specifying the content-type
explicitly.

A follow-up to ruby/rubygems@b70c1bb1503d

ruby/rubygems@d573af07f6

Author: mame

test/rubygems/utilities.rb

@@ -115,6 +115,7 @@ def request(uri, request_class, last_modified = nil)
 
     # Ensure multipart request bodies are generated
     socket = FakeSocket.new
+    last_request.content_type ||= "application/x-www-form-urlencoded"
     last_request.exec socket.binmode, "1.1", last_request.path
     _, last_request.body = socket.string.split("\r\n\r\n", 2)