[Fix #7639] Values for new attributes not saved (Blocker)

The typenames of created fields do not always match
(e.g. 
"Integer" vs. "integer"
"real" vs. "double"
)

Apparently
QgsVectorDataProvider::addAttributes() does not produce the same as
QgsVectorDataProvider::nativeTypes() returns

Maybe this should be fixed in a different way?