Make TagHelperOutput.Attributes case insensitive.

- Changed the Attributes dictionary to utilize a StringComparer.OrdinalIgnoreCase comparer.
- Added a test to validate the new attribute mechanic.

#186
nit: suggest a `[Theory]` with `originalName` and `updateName` parameters to the test method. that'll let you check mixes such as `{ "CLASS", "class" }` (the opposite of current test), `{ "cLASS", "Class" }`, ...