KeyTag for RSAMD5 seems to be wrong

[ulrichwisser]

I have tried to create keys with keytag collision. While doing this I found that this golang dns library seems to compute false keytags for these keys.
I have created the keys with ldns and ldns computes the same keytag for both keys. I have loaded the keys into knot and retrieved with kdig and kdig computes the same keytag for both keys and the same as ldns.
Keytags for other algorithms are correct.
I have written a short test script to show the error.

package main

import (
	"fmt"

	"github.com/miekg/dns"
)

func main() {
	rr1, _ := dns.NewRR("test. IN      DNSKEY  257 3 13 +s9JyBVaDVllZjVM1sgpXZxjo13cz/KHiYfQ0P14BaUYcy8/L8b57AU3AoZ0/Ken5lG+ZO9biPC8Ek+XVn55BQ== ;{id = 185 (ksk), size = 256b}")
	rr2, _ := dns.NewRR("test. IN      DNSKEY  257 3 13 muZog6Q/E0r76zzPY7sN6hlE/LyKwaw/ymjhlBUbN6pTjm0DlQ5YkxjhliihD8wGJ0Jc35y0ETzDZSMEyA6msg== ;{id = 185 (ksk), size = 256b}")
	rr3, _ := dns.NewRR("test.	IN	DNSKEY	257 3 1 AwEAAcntNdoMnY8pvyPcpDTAaiqHyAhf53XUBANq166won/fjBFvmuzhTuP5r4el/pV0tzEBL73zpoU48BqF66uiL+qRijXCySJiaBUvLNll5rpwuduAOoVpmwOmkC4fV6izHOAx/Uy8c+pYP0YR8+1P7GuTFxgnMmt9sUGtoe+la0X/ ;{id = 27461 (ksk), size = 1024b}")
	rr4, _ := dns.NewRR("test.	IN	DNSKEY	257 3 1 AwEAAf0bKO/m45ylk5BlSLmQHQRBLx1m/ZUXvyPFB387bJXxnTk6so3ub97L1RQ+8bOoiRh3Qm5EaYihjco7J8b/W5WbS3tVsE79nY584RfTKT2zcZ9AoFP2XLChXxPIf/6l0H9n6sH0aBjsG8vabEIp8e06INM3CXVPiMRPPeGNa0Ub ;{id = 27461 (ksk), size = 1024b}")

	fmt.Printf("Key 1: keytag is %5d, expected   185\n", rr1.(*dns.DNSKEY).KeyTag())
	fmt.Printf("Key 2: keytag is %5d, expected   185\n", rr2.(*dns.DNSKEY).KeyTag())
	fmt.Printf("Key 3: keytag is %5d, expected 27461\n", rr3.(*dns.DNSKEY).KeyTag())
	fmt.Printf("Key 4: keytag is %5d, expected 27461\n", rr4.(*dns.DNSKEY).KeyTag())
}

output on my system is

Key 1: keytag is   185, expected   185
Key 2: keytag is   185, expected   185
Key 3: keytag is 17919, expected 27461
Key 4: keytag is 17691, expected 27461