Limitations for files with changing source

- What did you do?

Diff two already generated catalogs against each other by using the API functionality:

```
catalog_old.write(getCatalog(@options.merge({:puppetdb_url => 'https://puppetdb-live.example.org:8081'}), logger))
catalog_new.write(getCatalog(@options.merge({:puppetdb_url => 'https://puppetdb-dev.example.org:8081'}), logger))

catalog_diff_result = OctocatalogDiff::API::V1.catalog_diff(
{
:to_catalog => catalog_new.path,
:from_catalog => catalog_old.path,
}
)

diff_printer = OctocatalogDiff::Cli::Printer.new(@options, logger)
diff_printer.printer(catalog_diff_result.diffs)
```
Resource that changed looked like this:

```
file { "${nagios::nrpe::plugindir}/check_ram":
owner => 'root',
group => 'root',
mode => '0555',
source => 'puppet:///modules/ve_base/check_ram',
}
```
The content of the source file "ve_base/check_ram" changed.

- What happened?

octocatalog-diff didn't show any changes

- What did you expect to happen?

Show diffs on the changed content of the file

- How can someone reproduce the problem?

Generate two catalogs with a file {} resource which source file changed.

> Command used and debugging output

> Platform and version information

- Your OS:
Ubuntu 14.04

- Your Ruby version:
2.4.0
- Your version of Puppet:
4.10.1
- Your version of octocatalog-diff:
1.0.1
> Do the tests pass from a clean checkout?
Installed from gem, so hopefully ;)

> Anything else to add that you think will be helpful?

I think this has to do with how Puppet itself operates. Content of a file {} source parameter is retrieve while applying the catalog. My question therefor is: Are you aware of this limitation? Do you know any workaround which could help detect the change?