Fix charset handling in Zotero.File.getContentsAsync()

Character sets are now populated on demand, so they can't be run through
Zotero.CharacterSets.getName() in getContentsAsync(), since they might
not exist. (I'm also not sure why this was being done anyway.)

chrome/content/zotero/xpcom/file.js

@@ -209,7 +209,7 @@ Zotero.File = new function(){
 		}
 		
 		var options = {
-			charset: charset ? Zotero.CharacterSets.getName(charset) : "UTF-8",
+			charset: charset ? charset : "UTF-8",
 			// This doesn't seem to work -- reading an image file still throws NS_ERROR_ILLEGAL_INPUT
 			replacement: "\uFFFD"
 		};