Update `MapBinder` to only compute values if an entry does not already
exist. Prior to this commit, a binding to `Map<String,List<String>>`
would be expensive since the same entries would be bound many times.

For example, given:

	foo.bar[0]=baz1
	foo.bar[1]=baz1
	foo.bar[2]=baz1

The Map binder would iterate over the properties `bar[0]`, `bar[1]` and
`bar[2]`. Each of these properties resulted in the same actual key of
`bar` which would then be bound to list multiple times.

Fixes gh-10093