Add coverage of Has in test.

btree_test.go

diff --git a/btree_test.go b/btree_test.go
index feed087..2ff15bc 100644
--- a/btree_test.go
+++ b/btree_test.go
@@ -91,6 +91,11 @@
 			}
 		}
 		for _, item := range perm(treeSize) {
+			if !tr.Has(item) {
+				t.Fatal("has did not find item", item)
+			}
+		}
+		for _, item := range perm(treeSize) {
 			if x := tr.ReplaceOrInsert(item); x == nil {
 				t.Fatal("insert didn't find item", item)
 			}