Change Razor attribute behavior to not special case boolean values in middle of complex attribute values.

[NTaylorMullen]

Today if a user writes:

@{
    var isActive = true;
}
<input type="button" onclick="doSomething(@isActive);" />

It generates <input type="button" onclick="doSomething(onclick);" />. If isActive is false it removes the content entirely. This feature had good intentions in MVC5 but should be scoped to non-complex attribute values; aka values that only have a single value.

Intended output:

<input checked="@true" />                 => <input checked="checked" />
<input checked="@false" />                => <input />
<input checked="   @true         " />     => <input checked="   True         " />
<input checked="   @false        " />     => <input checked="   False        " />
<input checked="value: @true" />          => <input checked="value: True" />
<input checked="value: @false" />         => <input checked="value: False" />

[NTaylorMullen]

/cc @Eilon