repl: don't override all internal repl defaults

The createInternalRepl() module accepts an options object as an
argument. However, if one is provided, it overrides all of the
default options. This commit applies the options object to the
defaults, only changing the values that are explicitly set.

PR-URL: https://github.com/nodejs/node/pull/7826


Reviewed-By: James M Snell <jasnell@gmail.com>