State and country plugins should allow already valid abbreviations to pass through

The state and country conversion plugins in this module are a great feature, but it seems kind of silly that if the input data is already using the correct country code or state abbreviation, the plugin will remove the data rather than just letting it go.

In other words, currently these plugins will allow "United States" and "Maryland" (and convert them to "US" and "MD" for you) but reject "US" and "MD" themselves. I think they should allow both.