Refs #412. Fixed issue where td with colspan would lose its colspan when switched to a th by the user.
Refs #412. Added tests for preserving the colspan when the user switches td and th containers.
Refs 412. Updated CHANGELOG.
Refs #412. Improved layout of tests and made switchTo more efficient.
Merge pull request #412 from FriedRice/issue_412

Fixes #412. Converting td's to th's strips colspan/rowspan