[api-minor] Change the dc:subject Metadata field to an Array

This patch simply extends the existing handling of the `dc:creator` field, which should hopefully suffice here; please refer to https://wwwimages2.adobe.com/content/dam/acom/en/devnet/xmp/pdfs/XMP%20SDK%20Release%20cc-2016-08/XMPSpecificationPart1.pdf#page=34

Author: Snuffleupagus

src/display/metadata.js

@@ -89,22 +89,24 @@ class Metadata {
     return entry.childNodes.filter(node => node.nodeName === "rdf:li");
   }
 
-  _getCreators(entry) {
-    if (entry.nodeName !== "dc:creator") {
-      return false;
-    }
+  _getArray(entry) {
+    const name = entry.nodeName;
+    assert(
+      name === "dc:creator" || name === "dc:subject",
+      `_getArray - unexpected type: "${name}".`
+    );
+
     if (!entry.hasChildNodes()) {
       return true;
     }
-
     // Child must be a Bag (unordered array) or a Seq.
-    const seqNode = entry.childNodes[0];
+    const [seqNode] = entry.childNodes;
     const authors = this._getSequence(seqNode) || [];
+
     this._metadataMap.set(
-      entry.nodeName,
+      name,
       authors.map(node => node.textContent.trim())
     );
-
     return true;
   }
 
@@ -130,11 +132,15 @@ class Metadata {
 
       for (const entry of desc.childNodes) {
         const name = entry.nodeName;
-        if (name === "#text") {
-          continue;
-        }
-        if (this._getCreators(entry)) {
-          continue;
+        switch (name) {
+          case "#text":
+            continue;
+          case "dc:creator":
+          case "dc:subject":
+            if (this._getArray(entry)) {
+              continue;
+            }
+            break;
         }
         this._metadataMap.set(name, entry.textContent.trim());
       }

test/unit/metadata_spec.js

@@ -171,7 +171,7 @@ describe("metadata", function () {
       "dc:creator": [""],
       "dc:description": "",
       "dc:format": "application/pdf",
-      "dc:subject": "",
+      "dc:subject": [],
       "dc:title": "",
       "pdf:keywords": "",
       "pdf:pdfversion": "1.7",