fx-compat: OS.File.read() → IOUtils.read() in getContentsAsync()

https://firefox-source-docs.mozilla.org/dom/ioutils_migration.html

This also fixes a bug when `getContentsAsync()` is passed an
`nsIInputStream` or `nsIChannel` where raw bytes were returned instead
of a string. Not sure if we're doing that anywhere. If we are, this
would presumably break that code, but the function is supposed to return
a decoded string.

test/tests/fileTest.js

@@ -36,7 +36,7 @@ describe("Zotero.File", function () {
 				OS.Path.join(getTestDataDirectory().path, "charsets", "invalid.txt")
 			);
 			assert.lengthOf(contents, 3);
-			assert.equal(contents, "A\uFFFDB");
+			assert.equal(contents, "A" + Zotero.File.REPLACEMENT_CHARACTER + "B");
 		})
 		
 		it("should respect maxLength", function* () {