fix: return correct family from toOptions with dns4 MultiAddr (#233)

When calling toOptions with a dns4 multiaddress, a MultiaddrObject is returned where the ip family is 6 instead of 4. This causes an issue where libp2p is unable to resolve the domain and dial the peer. This only appears to be an issue on some systems. It works on Mac OS but fails on for example Alpine Linux.

Example:
```javascript
const {multiaddr} = require("multiaddr")

console.log(multiaddr('/dns4/google.net/tcp/8000').toOptions())`
```

```javascript
Object {family: 6, host: "google.net", transport: "tcp", port: 8000}
```

Co-authored-by: achingbrain <alex@achingbrain.net>

src/index.ts

@@ -141,13 +141,32 @@ export class Multiaddr {
    * ```
    */
   toOptions (): MultiaddrObject {
-    const parsed = this.toString().split('/')
+    const codes = this.protoCodes()
+    const parts = this.toString().split('/').slice(1)
+    let transport: string
+    let port: number
+
+    if (parts.length > 2) {
+      // default to https when protocol & port are omitted from DNS addrs
+      if (DNS_CODES.includes(codes[0]) && P2P_CODES.includes(codes[1])) {
+        transport = getProtocol('tcp').name
+        port = 443
+      } else {
+        transport = getProtocol(parts[2]).name
+        port = parseInt(parts[3])
+      }
+    } else if (DNS_CODES.includes(codes[0])) {
+      transport = getProtocol('tcp').name
+      port = 443
+    } else {
+      throw new Error('multiaddr must have a valid format: "/{ip4, ip6, dns4, dns6, dnsaddr}/{address}/{tcp, udp}/{port}".')
+    }
 
     const opts: MultiaddrObject = {
-      family: parsed[1] === 'ip4' ? 4 : 6,
-      host: parsed[2],
-      transport: parsed[3],
-      port: parseInt(parsed[4])
+      family: (codes[0] === 41 || codes[0] === 55) ? 6 : 4,
+      host: parts[1],
+      transport,
+      port
     }
 
     return opts

test/index.spec.ts

@@ -486,6 +486,46 @@ describe('helpers', () => {
           port: 1234
         })
     })
+
+    it('returns an options object from a DNS addr', () => {
+      expect(new Multiaddr('/dns4/google.net/tcp/8000').toOptions())
+        .to.eql({
+          family: 4,
+          host: 'google.net',
+          transport: 'tcp',
+          port: 8000
+        })
+    })
+
+    it('returns an options object from a DNS6 addr', () => {
+      expect(new Multiaddr('/dns6/google.net/tcp/8000').toOptions())
+        .to.eql({
+          family: 6,
+          host: 'google.net',
+          transport: 'tcp',
+          port: 8000
+        })
+    })
+
+    it('returns an options object from a DNS addr defaulting to https', () => {
+      expect(new Multiaddr('/dnsaddr/google.net').toOptions())
+        .to.eql({
+          family: 4,
+          host: 'google.net',
+          transport: 'tcp',
+          port: 443
+        })
+    })
+
+    it('returns an options object from a DNS addr with a PeerID defaulting to https', () => {
+      expect(new Multiaddr('/dnsaddr/google.net/p2p/QmcgpsyWgH8Y8ajJz1Cu72KnS5uo2Aa2LpzU7kinSupNKC').toOptions())
+        .to.eql({
+          family: 4,
+          host: 'google.net',
+          transport: 'tcp',
+          port: 443
+        })
+    })
   })
 
   describe('.inspect', () => {