Zotero.HTTP.request(): Process headers case insensitively

Using the Headers class from the Fetch API.

Before, the added test would fail: `_requestInternal()`, not finding a header
named `Content-Type` (case sensitive), would set it to
`application/x-www-form-urlencoded`. XMLHttpRequest, upon being given both
`content-type`: `application/json`) and `Content-Type`:
`application/x-www-form-urlencoded`, would helpfully merge the two, producing
`content-type`: `application/json, application/x-www-form-urlencoded`. That's
obviously not the correct behavior.

test/tests/httpTest.js

@@ -45,6 +45,20 @@ describe("Zotero.HTTP", function () {
 				}
 			}
 		);
+		httpd.registerPathHandler(
+			'/requireJSON',
+			{
+				handle(request, response) {
+					if (request.getHeader('Content-Type') == 'application/json') {
+						response.setStatusLine(null, 200, "OK");
+					}
+					else {
+						response.setStatusLine(null, 400, "Bad Request");
+					}
+					response.write('JSON required');
+				}
+			}
+		);
 	});
 	
 	beforeEach(function () {
@@ -124,6 +138,20 @@ describe("Zotero.HTTP", function () {
 			server.respond();
 		});
 		
+		it("should process headers case insensitively", async function () {
+			Zotero.HTTP.mock = null;
+			var req = await Zotero.HTTP.request(
+				'GET',
+				baseURL + 'requireJSON',
+				{
+					headers: {
+						'content-type': 'application/json'
+					}
+				}
+			);
+			assert.equal(req.status, 200);
+		});
+		
 		describe("Retries", function () {
 			var spy;
 			var delayStub;