TagHelperOutput doesn't allow multiple non-bound TagHelper attributes with identical names.

[NTaylorMullen]

This item adds the infrastructure needed to complete #220 and https://github.com/aspnet/Razor/issues/96.

Ultimately we should be able to have tags such as:
<input class="btn" class="btn-success" /> which generate the correct output. Today this type of tag which happens to be a TagHelper would generate:
<input class="btn-success" /> because last attribute in wins with no way for the TagHelper author to chose the other value.

To properly complete this item we need to create an attribute abstraction that has many types, one of which is a one key => many values (in addition to the default 1=>1 mapping). Other types that could be added in the future are key only, C# etc.

[NTaylorMullen]

Assigned this to RC due to #220 and #96 being assigned to the milestone (this is a pre-requisite).

[dougbu]

@NTaylorMullen, could you clarify why this issue is a prerequisite to #220 (single attributes w/ no value e.g. checked) or #96 (single C# attributes e.g. @checked)?

[NTaylorMullen]

The correct way to fix #96/#220 is to enable different types of attribute values in the TagHelperOutput. Any other fix would be more or less a hack/half-fix. By having a single attribute abstraction that can handle 1->many, no value, 1->1, C# attributes users then only have a single abstraction they need to work with without risk of ambiguity between things like:

<input checked="" /> and <input checked />