Edited Feature: [EditorTemplates] Support htmlAttributes as IDictionary for dynamic scenarios [1687]

I can do this:

```csharp
@Html.EditorFor(m => m.Foo, new { htmlAttributes = new { @class = "form-control" } })
```

but I cannot do this:

```csharp
@Html.EditorFor(m => m.Foo, new { htmlAttributes = new Dictionary<string, object> { { "class", "form-control" } } })
```

Even though with the low-level helpers I can do both:

```csharp
@Html.TextBoxFor(m => m.Foo, new { @class = "form-control" })
@Html.TextBoxFor(m => m.Foo, new Dictionary<string, object> { { "class", "form-control" } })
```