Allow charRange.choose to pick the last rune

Before this change, choose would never pick the last character of the set. This
is not the expected behavior: if the range is a-z, we expect a,b,...,y,z to be
picked in a uniform way.

fuzz.go

@@ -482,7 +482,7 @@ type charRange struct {
 // choose returns a random unicode character from the given range, using the
 // given randomness source.
 func (cr charRange) choose(r int63nPicker) rune {
-	count := int64(cr.last - cr.first)
+	count := int64(cr.last - cr.first + 1)
 	return cr.first + rune(r.Int63n(count))
 }
 

fuzz_test.go

@@ -533,8 +533,8 @@ func Test_charRange_choose(t *testing.T) {
 	t.Run("Picks last", func(t *testing.T) {
 		r := alwaysLast{}
 		letter := lowercaseLetters.choose(r)
-		if letter != 'y' {
-			t.Errorf("Expected y, got %v", letter)
+		if letter != 'z' {
+			t.Errorf("Expected z, got %v", letter)
 		}
 	})
 }