API Workflow Invocation replacement_params only accepts strings

POST /api/workflows/{encoded_workflow_id}/invocations

```
{
...
replacement_params: { "foo" : 2 }
}
```

quietly fails to do the replacement.

The following instead works:

```
{
...
replacement_params: { "foo" : "2" }
}
```

Start by tracing the POST /api/workflows/{encoded_workflow_id}/invocations endpoint and how it handles replacement_params. Reproduce the difference between numeric and string values, then add coverage for numeric replacement_params and confirm both forms perform the replacement.

Written by the indexing model from the issue text.