Feature/gh 5 - Githubissues

Closes #5

This change:

adds tests of enumeration detection on empty strings. (The actual fix for handling empty strings was done in a previous change to mdm-plugin-database https://github.com/MauroDataMapper-Plugins/mdm-plugin-database/pull/10)
adds a test that summary metadata is saved to the data class as well as data element (the actual save is done in the above mentioned PR to mdm-plugin-database)
adds nchar and nvarchar to the column data types which trigger enumeration detection

MauroDataMapper-Plugins / mdm-plugin-database-sqlserver