fix(pdk) treat custom content blocks as the Admin API
fix(pdk) treat custom content blocks as the Admin API

This is a small workaround to a problem that happens when any custom
content_by_lua block attempts to use the PDK. For example, this happens when
trying to hit the `/` endpoint of the metrics server of the Prometheus plugin:

https://github.com/Kong/kong-plugin-prometheus/blob/0.4.1/kong/plugins/prometheus/serve.lua#L32

In normal operation of Kong, `kong.ctx.core.phase` is always set,
so the code path added should never be hit.

Kong/kong,Kong/kong,Mashape/kong,Kong/kong