Fix HTTP.request() 5xx handling with errorDelayMax=0

This was causing delay-less infinite loops for OPTIONS requests to Box,
which are returning 502 errors.

chrome/content/zotero/xpcom/http.js

@@ -166,8 +166,11 @@ Zotero.HTTP = new function() {
 						if (e.xmlhttp.status == 503 && await _checkRetry(e.xmlhttp)) {
 							continue;
 						}
+						// Don't retry if errorDelayMax is 0
+						if (options.errorDelayMax === 0) {
+							throw e;
+						}
 						// Automatically retry other 5xx errors by default
-						if (options.errorDelayMax !== 0) {
 						if (!errorDelayGenerator) {
 							// Keep trying for up to an hour
 							errorDelayGenerator = Zotero.Utilities.Internal.delayGenerator(
@@ -206,7 +209,6 @@ Zotero.HTTP = new function() {
 							Zotero.logError("Failed too many times");
 							throw e;
 						}
-						}
 						continue;
 					}
 				}

test/tests/httpTest.js

@@ -169,6 +169,29 @@ describe("Zotero.HTTP", function () {
 				assert.equal(delayStub.args[1][0], 20);
 			});
 			
+			it("shouldn't retry on 500 error if errorDelayMax=0", async function () {
+				setResponse({
+					method: "GET",
+					url: "error",
+					status: 500,
+					text: ""
+				});
+				spy = sinon.spy(Zotero.HTTP, "_requestInternal");
+				var e = await getPromiseError(
+					Zotero.HTTP.request(
+						"GET",
+						baseURL + "error",
+						{
+							errorDelayIntervals: [10, 20, 100],
+							errorDelayMax: 0
+						}
+					)
+				);
+				assert.instanceOf(e, Zotero.HTTP.UnexpectedStatusException);
+				assert.isTrue(spy.calledOnce);
+				assert.isTrue(delayStub.notCalled);
+			});
+			
 			it("should provide cancellerReceiver a callback to cancel while waiting to retry a 5xx error", async function () {
 				delayStub.restore();
 				setResponse({