add regression test for NaN handling in errorbar mode.

See https://code.google.com/p/dygraphs/issues/detail?id=466 .

The reported issue was that NaN values were unexpectedly being
drawn as connected in customBar=true mode. This already
works correctly in current Dygraphs, I've converted the jsfiddle
demonstrating the issue to a regression test.
Merge pull request #284 from klausw-g/issue-466-regression-test

add regression test for NaN handling in errorbar mode.