Routes parsing breaks when combining a scala keyword with a default value

Found on play v2.2.1. Both features work fine by themselves, but break when combined. Note the parameter "type".

Using a scala keyword with a fixed value (or no value) gets escaped correctly and works:
```scala
  GET       /designer     controllers.Designer.index(type = "x") 
```
Using a default value with "type" renamed to "tpe" (non-keyword) works (of course): 
```scala
  GET       /designer     controllers.Designer.index(tpe ?= "x")
```

But in combination:
```scala
  GET       /designer     controllers.Designer.index(type ?= "x")
```
produces
```sbt
[error] /Users/alvarocarrasco/workspace/devstack/conf/routes:26: illegal start of simple expression
[error] GET       /designer    controllers.Designer.index(type ?= "x")
[error] /Users/alvarocarrasco/workspace/devstack/conf/routes:26: ')' expected but '}' found.
[error] GET       /designer    controllers.Designer.index(type ?= "x")
[error] two errors found
```

Tried looking at the source, but couldn't spot the cause.