Issue with /webhook calls failing when called from curated-data and curated-data-dev

# Problem with /webhook calls failing from curated-data repos

I debated whether to file this in this repo or the curated-data repo, but I believe this is an issue on the service side.

## Background

Whenever a pull request is open, merged, or closed in the [curated-data repo](https://github.com/clearlydefined/curated-data), this [webhook](https://github.com/clearlydefined/curated-data/settings/hooks) fires (you may need admin access to the curated-data repo to see the webhook config).

Here is the code for that [webhook](https://github.com/clearlydefined/service/blob/424386d0febdc3ed267dc8792db6eb8d3ba17a75/routes/webhook.js#L25).

## Problem

Looking at the recent deliveries of that webhook, [there are several failures](https://github.com/clearlydefined/curated-data/settings/hooks/22795049/deliveries).

Interestingly, we’re seeing similar activity in the same webhook on the [curated-data-dev repo](https://github.com/clearlydefined/curated-data-dev/settings/hooks/19525747/deliveries).

I was also able to replicate this locally using [the Docker dev environment setup](https://github.com/clearlydefined/docker_dev_env_experiment) and a [forked copy of the curated-data-dev repo](https://github.com/nellshamrell/curated-data-dev).

## Research

After some experimentation, it appears the webhook is working for pushes to the curated-data and curated-data-dev repos, but not pull requests (the webhook on each respective repo is configured to run on both of those types of events).

I'm not sure why this is, but am wondering if the ClearlyDefined service is rejecting webhooks with pull request payloads for some reason, but not webhooks with push payloads.