g8Test failed when using Formatting template fields for filename

Envilonment
- Windows 7
- sbt version: 1.1.0

Repoducing procedures is as follows:

- sbt new foundweekends/giter8.g8
- use all default value
- cd my-template-project
- sbt g8Test (Works fine)
- change src/main/scala/stub.scala to src/main/scala/$name$.scala
- Then, execute again "sbt 8Test", but it failed with following messaages.

```
> sbt g8Test
Picked up JAVA_TOOL_OPTIONS: -Djava.vendor="Sun Microsystems, Inc"
Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
[info] Loading settings from plugins.sbt ...
[info] Loading project definition from C:\work\prog\tmp\my-template-project\project
[info] Loading settings from build.sbt ...
[info] Set current project to My Template Project (in build file:/C:/work/prog/tmp/my-template-project/)
[error] org.stringtemplate.v4.compiler.STException: relative: src\main\scala\$name$.scala, toPath: C:\work\prog\tmp\my-template-project\target\sbt-test\My Template Project\scripted, 1:20: doesn't look like an expression
[error] at giter8.G8$.expandPath(g8.scala:201)
[error] at giter8.G8$.$anonfun$apply$2(g8.scala:98)
[error] at scala.collection.TraversableLike.$anonfun$flatMap$1(TraversableLike.scala:241)
[error] at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:59)
[error] at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:52)
[error] at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
[error] at scala.collection.TraversableLike.flatMap(TraversableLike.scala:241)
[error] at scala.collection.TraversableLike.flatMap$(TraversableLike.scala:238)
[error] at scala.collection.AbstractTraversable.flatMap(Traversable.scala:104)
[error] at giter8.G8$.apply(g8.scala:96)
[error] at giter8.Giter8Plugin$.$anonfun$giter8TestSettings$6(Giter8Plugin.scala:105)
[error] at scala.Function1.$anonfun$compose$1(Function1.scala:44)
[error] at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:39)
[error] at sbt.std.Transform$$anon$4.work(System.scala:66)
[error] at sbt.Execute.$anonfun$submit$2(Execute.scala:262)
[error] at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:16)
[error] at sbt.Execute.work(Execute.scala:271)
[error] at sbt.Execute.$anonfun$submit$1(Execute.scala:262)
[error] at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:174)
[error] at sbt.CompletionService$$anon$2.call(CompletionService.scala:36)
[error] at java.util.concurrent.FutureTask.run(Unknown Source)
[error] at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
[error] at java.util.concurrent.FutureTask.run(Unknown Source)
[error] at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
[error] at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
[error] at java.lang.Thread.run(Unknown Source)
[error] (Test / g8) org.stringtemplate.v4.compiler.STException: relative: src\main\scala\$name$.scala, toPath: C:\work\prog\tmp\my-template-project\target\sbt-test\My Template Project\scripted, 1:20: doesn't look like an expression
[error] Total time: 1 s, completed 31-Jan-2018 17:53:42
```

However, it works fine to use this template as follows

sbt new file://./my-template-project/