Rename http.resend_count to http.request.resend_count

[trask]

Changes

Renames http.resend_count to http.request.resend_count to be consistent with http.request.method, http.response.status_code, and other http attributes which are generally prefixed with http.request.* or http.response.*.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.