[BUG] Trigger's spec.attributes does not work with numbers

[Harwayne]

Describe the bug
Trigger's spec.attributes is a map[string]string, for exact matching with CloudEvents attributes. If the value of the attribute happens to look like a number (e.g. 42), then you cannot match it. As it is considered a number, which is never equal to a string.

Expected behavior
If spec.attributes is { the: "42" }, and I send a CloudEvent with an attribute in HTTP binary content mode with an HTTP header of ce-the: 42, then I expect the filter to match.

To Reproduce

  apiVersion: eventing.knative.dev/v1alpha1
  kind: Trigger
  spec:
    broker: default
    filter:
      attributes:
        heart: "yes"
        source: https://github.com/knative/eventing-sources/cmd/heartbeats/#event-test/mypod
        the: "42"
        type: dev.knative.eventing.samples.heartbeat
    subscriber:
      ref:
        apiVersion: v1
        kind: Service
        name: event-display
---
  apiVersion: sources.eventing.knative.dev/v1alpha1
  kind: ContainerSource
  metadata:
    name: test-heartbeats
    namespace: default
  spec:
    args:
    - --period=1
    env:
    - name: POD_NAME
      value: mypod
    - name: POD_NAMESPACE
      value: event-test
    image: index.docker.io/daisyycguo/heartbeats-6790335e994243a8d3f53b967cdd6398
    sink:
      apiVersion: eventing.knative.dev/v1alpha1
      kind: Broker
      name: default

[grantr]

Is this still an issue now that CloudEvent extensions are always strings?