test(sankey): collect all Lib.warn arguments in the spy

Lib.warn is variadic; pushing only the first argument made
warnings[0][0] a single character instead of the message prefix.

Author: CAOShurong

test/jasmine/tests/sankey_test.js

@@ -127,8 +127,9 @@ describe('sankey tests', function() {
 
         it('warns when the figure is too small for node.pad', function(done) {
             var warnings = [];
-            spyOn(Lib, 'warn').and.callFake(function(msg) {
-                warnings.push(msg);
+            spyOn(Lib, 'warn').and.callFake(function() {
+                // collect all arguments, as Lib.warn is variadic
+                warnings.push(Array.prototype.slice.call(arguments));
             });
 
             var fig = Lib.extendDeep({}, padMock);