Fix binding of overlapping nested maps

When binding a nested map structure, RelaxedDataBinder pre-populates
the target object with default empty maps. Previously, when these
structures overlapped, each step in pre-population process could
potentially overwrite what had come before it. This led to the output
of the pre-population process being incomplete which would lead to a
binding failure.

This commit updates the pre-population process so that it checks to see
if a property's value has already been set by an earlier step in the
process. If it has been set, the existing value is now reused rather
than being overwritten by a new empty map.

Fixes #328