Make `Radio` return arbitrary values instead of strings

[johroj]

My intuition suggests that Radio and Select should behave identically apart from visual appearance. However, Radio only supports strings as key values. Thus,

@bind M Radio([M1 => "Diagonal Matrix", M2 => "Symmetric Matrix"])

does not work but

@bind M Select([M1 => "Diagonal Matrix", M2 => "Symmetric Matrix"])

does.

It seems this issues used to exist for Select too, but it was fixed in #3. Was Radio perhaps forgotten?