fix: bring back content-type header

[javiermolinar]

What this PR does:
This pr brings back the application-json content-type header.
Previously the common combiner harcoded the header to application json. Now we need to initialize the combiner with default values

Which issue(s) this PR fixes:
Fixes #4121

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[github-actions]

The backport to release-v2.6 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-4123-to-release-v2.6 origin/release-v2.6
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 6afedd95e5b94d7f4dcac0175116d305ce4c619e

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-4123-to-release-v2.6
# Create the PR body template
PR_BODY=$(gh pr view 4123 --json body --template 'Backport 6afedd95e5b94d7f4dcac0175116d305ce4c619e from #4123{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title '[release-v2.6] fix: bring back content-type header' --body-file - --label 'type/bug' --label 'backport' --base release-v2.6 --milestone release-v2.6 --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-4123-to-release-v2.6

# Create a pull request where the `base` branch is `release-v2.6` and the `compare`/`head` branch is `backport-4123-to-release-v2.6`.

# Remove the local backport branch
git switch main
git branch -D backport-4123-to-release-v2.6