Postponing copying of not-configurable PROPERTY_HANDLER properties.

Postponing copying of not-configurable PROPERTY_HANDLER properties.

Since df385232d2af a shared property is copied to object hash on the
first access. It simplified changing of configurable shared properties.

Since 50fded8ccee5 Array.prototype functions treat empty array hash as a
sign that array instance is simple (without property getters and
non-numerical properties) to iterate over array values optimally.

Accessing "length" property made a copy in array hash. As a result next
iterations over array became not optimized.

The fix is to postpone making a mutable copy of not-configurable
NJS_PROPERTY_HANDLER properties until they are really required to change.

This closes #220 issue on Github.