Mapped commands aren't re-mapped

Ran into a weird one today, I had something like:

``` rub
SSHKit.config.command_map[:rake] = "/usr/local/rbenv/shims/bundle exec rake"
SSHKit.config.command_map[:ruby] = "/usr/local/rbenv/shims/ruby"
```

I had a problem with `Rake x.x.x is already activated, bundle exec might help` so I tried mapping:

```
SSHKit.config.command_map[:bundle] = "/usr/local/rbenv/shims/bundle"
```

Then I realised it would have been nice to be able to have have mapped:

```
SSHKit.config.command_map[:rake] = "bundle exec rake"
```

and have the `bundle` part of that be _somehow_ re-expanded.

In the end I worked around by using:

``` ruby
SSHKit.config.command_map[:bundle] = "/usr/local/rbenv/shims/bundle"
SSHKit.config.command_map[:rake] = "/usr/local/rbenv/shims/bundle exec rake"
SSHKit.config.command_map[:ruby] = "/usr/local/rbenv/shims/ruby"
```

Which is still absolutely OK, but weird that I never thought of this when designing the command map.