Auto Attribute removal in TagHelpers

[yishaigalatzer]

Attributes that in normal razor code gets automatically removed gets special treatment with TagHelpers.

1. Only attributes that are not bound to tag helper properties will keep the existing razor behavior.
2. These attributes will generate the "naive" string and populate the TagHelperContext.AllAttributes
3. Expression Values that are result in false, null or string.Empty are stripped from TagHelperOutput.Attributes.
   1. Example input:
      1. TagHelperContext.AllAttributes will include [class, "false"]
      2. TagHelperOutput.Attributes will not include class.
   2. For other examples follow unit tests for WriteAttribute.

[dougbu]

think we tested these scenarios and found attribute="@string.Empty" resulted in attribute="" with a regular (non-tag helper) attribute. only attribute="@null" and attribute="@false" lead to no generation for the attribute. tag helper attributes should do the same thing.

FYI, see special-cases at:

• https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs#L336 (for null)
• https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs#L358 (for false)