unmanagedSourceDirectories does not propagate

[rossabaker]

When copied into a projects project directory, this plugin sets unmanagedSourceDirectories correctly. When used as a plugin, it does not:

[root] λ inspect unmanagedSourceDirectories
[info] Setting: scala.collection.Seq[java.io.File] = List(/Users/bakro29/src/http4s/src/main/scala-2.12, /Users/bakro29/src/http4s/src/main/scala, /Users/bakro29/src/http4s/src/main/java)
[info] Description:
[info] 	Unmanaged source directories, which contain manually created sources.
[info] Provided by:
[info] 	{file:/Users/bakro29/src/http4s/}http4s/compile:unmanagedSourceDirectories
[info] Defined at:
[info] 	(sbt.Defaults) Defaults.scala:193

scalazVersion, from the same plugin, works just fine:

[root] λ inspect scalazVersion
[info] Setting: java.lang.String = 7.2.9
[info] Description:
[info] 	The version of Scalaz used for building.
[info] Provided by:
[info] 	{file:/Users/bakro29/src/http4s/}http4s/*:scalazVersion
[info] Defined at:
[info] 	(verizon.build.ScalazPlugin) ScalazPlugin.scala:36

What is different about unmanagedSourceDirectories that it doesn't override Defaults.scala?