Coerce multipart content type to a string before validating

The CR/LF check via Regexp#match? requires a String, but the per-part
content type was previously interpolated through to_s and accepted any
object such as a Symbol. Coerce it first to keep that behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

lib/net/http/generic_request.rb

@@ -349,7 +349,7 @@ def encode_multipart_form_data(out, params, opt)
       buf << "--#{boundary}\r\n"
       if filename
         filename = quote_string(filename, charset)
-        type = h[:content_type] || 'application/octet-stream'
+        type = (h[:content_type] || 'application/octet-stream').to_s
         if /[\r\n]/.match?(type)
           raise ArgumentError, "field content type cannot include CR/LF"
         end

test/net/http/test_http.rb

@@ -944,6 +944,9 @@ def test_set_form_multipart_crlf_injection
     assert_raise(ArgumentError) do
       build.call([['f', 'v', {filename: 'a', content_type: "text/plain\r\nX-Injected: 1"}]])
     end
+    assert_nothing_raised do
+      build.call([['f', 'v', {filename: 'a', content_type: :"text/plain"}]])
+    end
   end
 end