Address edge case for number_to_human with units option.

`ActiveSupport::NumberHelper#number_to_human` now returns the number unaltered when
the units hash does not contain the needed key, e.g. when the number provided is less
than the largest key provided.

Examples:

```
number_to_human(123, :units => {}) # => 123
number_to_human(123, :units => {:thousand => 'k'}) # => 123
```

Fixes #9269.


*Note: This pull request is a recision of #9311, which I abandoned due to source control problems.*