React to case insensitive TagHelperOutput.Attributes.

- Cleaned up some existing bad code that worked around case sensitive TagHelperOutput.Attributes.
- Modified MergeAttributes to be case insensitive when it comes to merging class attributes.
- Added a test to verify the new MergeAttribute case insensitive class merging.

https://github.com/aspnet/Mvc/issues/1449

**NOTE:** This PR depends on https://github.com/aspnet/Razor/pull/198 being merged.