improve Cogen instance. fix TODO in test

[xuwei-k]

scalaprops-magnolia/src/test/scala/scalaprops/ScalapropsMagnoliaTest.scala

Lines 36 to 40 in c5e510b

	xs.exists { f =>
	// TODO
	// testValues.map(f).distinct.size == testValues.size
	testValues.map(f).distinct.size >= 10
	}