Feature/gh 5

[aaronforshaw]

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 gh-9 Skip empty strings '' for enumerations. Save summary metadata to… mdm-plugin-database#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