[css-typed-om] custom properties should not be treated as case-insensitive

[dbaron]

The definition of StylePropertyMap says:

All properties stored on the property model in StylePropertyMapReadOnly are ASCII case-insensitive.

I think this is incorrect for custom properties, which should be case sensitive.

(Note that a few lines below, things are stated correctly:

1. If property does not start with two dashes (U+002D HYPHEN), let property be property ASCII lowercased.

)