Run the tests through bundle exec

Without it the tests load whatever net/protocol the Ruby ships rather
than the net-protocol the gemspec now requires, so on Ruby 2.7, where
net/protocol is plain stdlib and not a default gem, readuntil has no
limit keyword and every response read fails.

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

Author: hsbt

.github/workflows/test.yml

@@ -29,8 +29,8 @@ jobs:
       env:
         BUNDLE_WITHOUT: sig
     - name: Run test
-      run: rake test
+      run: bundle exec rake test
     - name: Build
-      run: rake build
+      run: bundle exec rake build
       env:
         LANG: C