Make Zotero.ItemFields.getFieldIDFromTypeAndBase() work on non-base fields

This previously returned false if a non-base-mapped field was passed,
even if the field was valid for the given item type. It now returns the
passed field as long as the field is valid for the type, which matches
the behavior for base fields.

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

@@ -260,6 +260,11 @@ Zotero.ItemFields = new function() {
 			throw new Error("Invalid field '" + baseField + '" for base field');
 		}
 		
+		// If field isn't a base field, return it if it's valid for the type
+		if (!this.isBaseField(baseFieldID)) {
+			return this.isValidForType(baseFieldID, itemTypeID) ? baseFieldID : false;
+		}
+		
 		return _baseTypeFields[itemTypeID][baseFieldID];
 	}