update the constants in MimeTypes to reflect current standards

[SingingBush]

mime types in barcode4j are pretty old and in some cases use the x- prefix that's used to indicate non-standard mime types. For example; the mime type for png currently used image/x-png instead of the image/png value as defined by the png ISO/IEC standard in 2004.

This can result in:

java.lang.UnsupportedOperationException: No BitmapEncoder available for image/x-png

To prevent any breaking changes for users tests should be in place to ensure that both the standardised and non-standard values work with the code.