Fix addLineClass (and similar) returning null on no-op

When the call didn't result in an actual change (for example
line already had the given class), changeLine would return null.

This doesn't seem useful, and caused issue #2438. It now always
returns the line handle (as documented).

Conflicts:
	lib/codemirror.js

Author: marijnh

lib/codemirror.js

@@ -2811,7 +2811,6 @@ window.CodeMirror = (function() {
     else no = lineNo(handle);
     if (no == null) return null;
     if (op(line, no)) regChange(cm, no, no + 1);
-    else return null;
     return line;
   }