Do not use mapping.to for strings.

Some packages (jshint, as an example) inject a `to` method into
`String.prototype`. This causes dynamo-table to attempt to use that
method for mapping.

It doesn't appear that there's any reason for dynamo-table to attempt to
use the `to` method of a String, so just short circuit the check.
Merge pull request #7 from remind101/master

Do not use mapping.to for strings.