Some operators against empty collection should throw UnsupportedOperationException

[xirc]

Closes the issue #137 😄

Operators below against empty collection should throw UnsupportedOperationException.

• reduce
• min
• max
• minBy
• maxBy

We can see scala.collection.iterator.reduce throws UnsupportedOperationException if the collection is empty.
scala.collection.iterator#reduce

Changes

• Nil.par.reduce throws UnsupportedOperationException.
• Nil.par.min throws UnsupportedOperationException.
• Nil.par.max throws UnsupportedOperationException.
• Add a test whether Nil.par.minBy throws UnsupportedOperationException.
• Add a test whether Nil.par.maxBy throws UnsupportedOperationException.

[SethTisue]

review by @scala/collections crew and/or by @axel22 ?

[johnynek]

Looks correct to me. Looks properly tested.

[SethTisue]

thank you @xirc, @johnynek, and @NthPortal!