knative - form data - support web action form data

OpenWhisk web actions may receive URL encoded form data `application/x-www-form-urlencoded` as input in addition to standard `application/json`.

V.S. Knative web actions only accepts `application/json` data. We need to find a way using [build file](https://github.com/mrutkows/openwhisk-knative-build/blob/master/runtimes/javascript/tests/webactionhelloworld/build.yaml.tmpl) to configure input data, may be with `arguments`:

https://github.com/mrutkows/openwhisk-knative-build/blob/master/runtimes/javascript/tests/webactionhelloworld/build.yaml.tmpl#L14