Framing to an id map (@container: @id) fails

[rybesh]

Describe the bug

A valid JSON-LD frame with an id map ("@container": "@id") fails with an error.

To Reproduce

The relevant test data and frame can be found on the JSON-LD playground.

The flattened JSON-LD graph and frame are loaded and processed using the following code:

    private static boolean frame(Path datasetPath, Path framePath) {
        try (BufferedReader datasetReader = Files.newBufferedReader(datasetPath);
                BufferedReader frameReader = Files.newBufferedReader(framePath)) {

            Document jsonld = JsonDocument.of(datasetReader);
            Document jsonldFrame = JsonDocument.of(frameReader);
            JsonObject results = JsonLd.frame(jsonld, jsonldFrame).get();

            Pretty.createWriter(System.out).writeObject(results);

            return true;

        } catch (IOException | JsonLdError e) {
            return err("Failed to frame dataset using %s: %s", framePath, e);
        }

Expected behavior

The reference JSON-LD processor at the Playground produces the expected output, but the code above (using the latest snapshot of titanium-json-ld) throws the following error: An @id entry was encountered whose value was not a string [code=INVALID_KEYWORD_ID_VALUE]

Additional context

This seems to be a separate issue from #171 since I get this error even when I load the flattened JSON-LD graph directly.

[rybesh]

Thank you! Working perfectly for me now.