[React] Allow default type parameter in users specified React component .

[yuit]

From discussion in here DefinitelyTyped/DefinitelyTyped#16112
Although we can't have generic users' specified React component as there are no way to specify type argument or infer such arguments, I think it should work with default type parameter

i.e below should be ok.

interface MapProps {
    prop1: string;
}
export class Map<P = MapProps, E = Leaflet.Map> extends MapComponent<P, E> { ... }
let x = <Map prop1="hi" />

Note this won't work

export class Map<P , E > extends MapComponent<P, E> { ... }