Fix saving session settings

As it stands, writing back *new* session settings that span more than 1 line will "double-space" each line inside the setting, not just between settings.
So something like 
```scala
val s: Setting[_]
val ss: SessionSettings.SessionSetting = (s, List(
  "libraryDependencies ++= Seq(",
  "  something,",
  "  something else",
  ")"
))
``` 
will end up written as
```scala
 libraryDependencies ++= Seq(

  something,

  something else

)
```

This fixes it to only add double-space *between* individual `SessionSetting`s.