Bug in type of `|` rule combinator

[alexander-myltsev]

parboiled2 type system should catch errors in a grammar as follows:

class SomeParser extends Parser {
  def test = rule { push("x") ~ (a | b) }
  def a = rule { str("a") ~> { (i: Int) => push("a") } }  
  def b = rule { str("b") ~> { (i: String) => push("b") } }
}

Currently it fails in runtime:

scala> new org.parboiled2.examples.SomeParser("a").test.run()
res0: scala.util.Try[String] = Failure(java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer)

scala> new org.parboiled2.examples.SomeParser("b").test.run()
res1: scala.util.Try[String] = Success(b)

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