Fix preact/debug accessing attributes that don't have toString

[pluscubed]

In some rare cases (e.g. Object.create(null)), a prop value doesn't have the toString() function, and thus cannot be cast to a string, throwing TypeError: Cannot convert object to primitive value. This should fix that.

[coveralls]

Coverage remained the same at 100.0% when pulling 8d34c7f on plusCubed:master into 824404b on developit:master.

[developit]

Looks good to me!

[developit]

I've looped @NekR in since he wrote this. Looks good to me though.

[coveralls]

Coverage remained the same at 100.0% when pulling 00be0a1 on plusCubed:master into e026d9e on developit:master.

[NekR]

Looks good for now.