fix: /api/forgejo/v1/version Content-Type error (#9897)

fix: #9782
## Checklist

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### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9897
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Cyborus <cyborus@noreply.codeberg.org>
Co-authored-by: fnoopv <fnoopv@noreply.codeberg.org>
Co-committed-by: fnoopv <fnoopv@noreply.codeberg.org>

routers/api/forgejo/v1/forgejo.go

@@ -18,6 +18,7 @@ func NewForgejo() *Forgejo {
 }
 
 func (f *Forgejo) GetVersion(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	w.WriteHeader(http.StatusOK)
 	_ = json.NewEncoder(w).Encode(Version{&setting.ForgejoVersion})
 }

tests/integration/api_forgejo_version_test.go

@@ -29,6 +29,13 @@ func TestAPIForgejoVersion(t *testing.T) {
 		assert.Equal(t, "1.0.0", *version.Version)
 	})
 
+	t.Run("Version with Content-Type is json", func(t *testing.T) {
+		req := NewRequest(t, "GET", "/api/forgejo/v1/version")
+		resp := MakeRequest(t, req, http.StatusOK)
+
+		assert.Equal(t, "application/json; charset=utf-8", resp.Header().Get("Content-Type"))
+	})
+
 	t.Run("Versions with REQUIRE_SIGNIN_VIEW enabled", func(t *testing.T) {
 		defer test.MockVariableValue(&setting.Service.RequireSignInView, true)()
 		defer test.MockVariableValue(&testWebRoutes, routers.NormalRoutes())()