Regression: Mapping labels fail to unmarshal

[psalaberria002]

Describe the bug
Mapping labels fail to unmarshal, making Ambassador crash on boot.

time="2021-01-07 19:11:40" level=error msg="goroutine \"/watcher\" exited with error: PANIC: json: cannot unmarshal array into Go struct field MappingSpec.spec.labels of type string" func="github.com/datawire/dlib/dgroup.(*Group).goWorkerCtx.func1.1" file="github.com/datawire/dlib@v1.1.0/dgroup/group.go:374" CMD=entrypoint PID=1 THREAD=/watcher

To Reproduce
Steps to reproduce the behavior:

1. Add the following Mapping

apiVersion: getambassador.io/v1
kind: Mapping
metadata:
  name: a
spec:
  prefix: /api/[^/]+/projects/[^/]+/a(/.*)?
  prefix_regex: true
  rewrite: ""
  service: "https://a"
  timeout_ms: 10000
  add_response_headers:
    Strict-Transport-Security: max-age=31536000
  retry_policy:
    retry_on: gateway-error
    num_retries: 15
  labels:
    ambassador:
      - d0:
          - a
          - xratelimitid:
              header: "X-Ratelimit-Id"
              omit_if_not_present: true

2. Apply the mapping

Expected behavior
It should be able to parse it. It's a valid config.

Versions (please complete the following information):

• Ambassador: 1.10 with AMBASSADOR_LEGACY_MODE=false