Fixed function "prototype" property handler while setting.

Fixed function "prototype" property handler while setting.

njs_function_prototype_create() works as a getter and setter.  As a
getter the function is expected to create "prototype" property on the
first access, it also sets F.prototype.constructor property to ensure
F.prototype.constructor === F.

Setting of "constructor" property is not needed in setter context, as
it may overwrite existing "constructor" property in setval.

This closes #333 issue on Github.