[All] Fix: mapWith in extras not being applied

[Angelelz]

close #1157

This will also help workaround #820 but more work will be needed to fully fix.

• Used the decoder passed to SQL columns defined in extras to the correct location when defining the correct alias for the column.
• Added integration tests.

To fix #820, using this PR, the user will have to manually cast the decimal value to char, similar to the implemented fix in #1688.

[DanielBaulig]

Any chance we can get this merged? I'm running into the issue described in #1157 too.

[benkingcode]

Please release this!