repl: support non-array `.scope`, document it

REPL evaluate `.scope` when it needs to get a list of the variable names
available in the current scope. Do not throw if the output of such
evaluation is not array, just ignore it.

PR-URL: https://github.com/nodejs/io.js/pull/1682


Reviewed-By: Fedor Indutny <fedor@indutny.com>