Handle non-string id blank node edge case.

In the case where an `@id` is not a string, consider an object a blank
node. This is needed to handle a crashing error where the code assumes
ids are valid strings. The error could occur with direct invalid input,
or in the edge case where non-normative invalid intermediate expanded
JSON-LD is generated.

Author: davidlehn

lib/graphTypes.js

@@ -106,11 +106,12 @@ api.isSimpleGraph = v => {
 api.isBlankNode = v => {
   // Note: A value is a blank node if all of these hold true:
   // 1. It is an Object.
-  // 2. If it has an @id key its value begins with '_:'.
+  // 2. If it has an @id key that is not a string OR begins with '_:'.
   // 3. It has no keys OR is not a @value, @set, or @list.
   if(types.isObject(v)) {
     if('@id' in v) {
-      return (v['@id'].indexOf('_:') === 0);
+      const id = v['@id'];
+      return !types.isString(id) || id.indexOf('_:') === 0;
     }
     return (Object.keys(v).length === 0 ||
       !(('@value' in v) || ('@set' in v) || ('@list' in v)));