Partially revert commit a64dc040.

Removing the conditional was too liberal as now attr() would even work
on plain javascript objects. Keeping a check to make sure it is at least
a DOM Node is appropriate. Using nodeName as in the commit which introduced
the nodeType === 1 check seems plausible.

src/attributes.js

@@ -206,7 +206,6 @@ jQuery.fn.extend({
 					return elem.getAttribute("value") === null ? "on" : elem.value;
 				}
 
-
 				// Everything else, we just grab the value
 				return (elem.value || "").replace(rreturn, "");
 
@@ -289,6 +288,8 @@ jQuery.extend({
 		// Try to normalize/fix the name
 		name = notxml && jQuery.props[ name ] || name;
 
+		// Only do all the following if this is a node (faster for style)
+		if ( elem.nodeName ) {
 			// These attributes require special treatment
 			var special = rspecialurl.test( name );
 
@@ -372,6 +373,7 @@ jQuery.extend({
 			// Non-existent attributes return null, we normalize to undefined
 			return attr === null ? undefined : attr;
 		}
+	}
 });
 
 })( jQuery );