Don't repeatedly upload relations

Fixes #2772

chrome/content/zotero/xpcom/data/dataObjectUtilities.js

@@ -275,7 +275,7 @@ Zotero.DataObjectUtilities = {
 		var pred2 = Object.keys(data2);
 		pred2.sort();
 		if (!Zotero.Utilities.arrayEquals(pred1, pred2)) return true;
-		for (let pred in pred1) {
+		for (let pred of pred1) {
 			let vals1 = typeof data1[pred] == 'string' ? [data1[pred]] : data1[pred];
 			let vals2 = (!data2[pred] || data2[pred] === '')
 				? []

test/tests/dataObjectUtilitiesTest.js

@@ -59,6 +59,42 @@ describe("Zotero.DataObjectUtilities", function() {
 			// place was already empty, so it shouldn't be included
 			assert.notProperty(obj, 'place');
 		});
+		
+		it("shouldn't include relations that haven't changed", function () {
+			var patchBase = {
+				title: "Old Title",
+				relations: {
+					"mendeleyDB:documentUUID": "6b97abe6-8e23-4471-b963-234cf26808b9"
+				}
+			};
+			var obj = {
+				title: "New Title",
+				relations: {
+					"mendeleyDB:documentUUID": "6b97abe6-8e23-4471-b963-234cf26808b9"
+				}
+			}
+			obj = Zotero.DataObjectUtilities.patch(patchBase, obj);
+			assert.notProperty(obj, 'relations');
+		});
+		
+		it("shouldn't include relations that only switched from string to array", function () {
+			var patchBase = {
+				title: "Old Title",
+				relations: {
+					"mendeleyDB:documentUUID": "6b97abe6-8e23-4471-b963-234cf26808b9"
+				}
+			};
+			var obj = {
+				title: "New Title",
+				relations: {
+					"mendeleyDB:documentUUID": [
+						"6b97abe6-8e23-4471-b963-234cf26808b9"
+					]
+				}
+			}
+			obj = Zotero.DataObjectUtilities.patch(patchBase, obj);
+			assert.notProperty(obj, 'relations');
+		});
 	})
 	
 	describe("#diff()", function () {