fails in postgres
```
*** (s.slick.compiler.QueryCompiler) After phase codeGen:
  ResultSetMapping : Coll[Option[String/VARCHAR(254)]]
    from s15: CompiledStatement "select max('test') from "u" s13 group by 'test'" : Coll[NominalType(t14)((Option[String/VARCHAR(254)]))]
    map: CompiledMapping : Option[String/VARCHAR(254)]

[error] Test com.typesafe.slick.testkit.tests.AggregateTest.testGroupBy[postgres] failed: org.postgresql.util.PSQLException: ERROR: non-integer constant in GROUP BY
[error]   Position: 42
[error]     at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2102)
[error]     at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1835)
[error]     at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:257)
[error]     at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:500)
[error]     at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:388)
[error]     at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:381)
[error]     at scala.slick.jdbc.StatementInvoker.results(StatementInvoker.scala:33)
[error]     at scala.slick.jdbc.StatementInvoker.iteratorTo(StatementInvoker.scala:16)
[error]     at scala.slick.jdbc.Invoker$class.foreach(Invoker.scala:93)
[error]     at scala.slick.jdbc.StatementInvoker.foreach(StatementInvoker.scala:9)
[error]     at scala.slick.jdbc.Invoker$class.build(Invoker.scala:69)
[error]     at scala.slick.jdbc.StatementInvoker.build(StatementInvoker.scala:9)
[error]     at scala.slick.jdbc.Invoker$class.to(Invoker.scala:77)
[error]     at scala.slick.jdbc.StatementInvoker.to(StatementInvoker.scala:9)
[error]     at scala.slick.driver.JdbcExecutorComponent$QueryExecutorDef.run(JdbcExecutorComponent.scala:23)
[error]     at scala.slick.driver.JdbcExecutorComponent$QueryExecutorDef.run(JdbcExecutorComponent.scala:15)
[error]     at com.typesafe.slick.testkit.tests.AggregateTest.testGroupBy(AggregateTest.scala:127)
[error]     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
[error]     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
[error]     at java.lang.reflect.Method.invoke(Method.java:597)
[error]     at com.typesafe.slick.testkit.util.Testkit$$anonfun$runChildren$2.apply(Testkit.scala:85)
[error]     at com.typesafe.slick.testkit.util.Testkit$$anonfun$runChildren$2.apply(Testkit.scala:76)
[error]     at scala.collection.TraversableLike$WithFilter$$anonfun$foreach$1.apply(TraversableLike.scala:772)
[error]     at scala.collection.Iterator$class.foreach(Iterator.scala:727)
[error]     at scala.collection.AbstractIterator.foreach(Iterator.scala:1157)
[error]     at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
[error]     at scala.collection.AbstractIterable.foreach(Iterable.scala:54)
[error]     at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:771)
[error]     at com.typesafe.slick.testkit.util.Testkit.runChildren(Testkit.scala:76)
[error]     at com.typesafe.slick.testkit.util.SimpleParentRunner.run(SimpleParentRunner.scala:56)
[error]     ...
```