Upgrade: Bump multipart-post from 2.0.0 to 2.1.0

Bumps [multipart-post](https://github.com/nicksieger/multipart-post) from 2.0.0 to 2.1.0.
- [Release notes](https://github.com/nicksieger/multipart-post/releases)
- [Changelog](https://github.com/socketry/multipart-post/blob/master/History.txt)
- [Commits](socketry/multipart-post@v2.0.0...v2.1.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Author: dependabot-support

Gemfile.lock

@@ -372,7 +372,7 @@ GEM
       ruby-progressbar
     multi_json (1.13.1)
     multi_xml (0.6.0)
-    multipart-post (2.0.0)
+    multipart-post (2.1.0)
     mustermann (1.0.3)
     mustermann-grape (1.0.0)
       mustermann (~> 1.0.0)