Fix ICECandidateStats.NetworkType type

Should be be marshaled/unmarshaled as a string to match the spec [0]

[0] https://www.w3.org/TR/2018/WD-webrtc-stats-20180627/#rtcnetworktype-enum

stats.go

@@ -1900,7 +1900,7 @@ type ICECandidateStats struct {
 	//
 	// DEPRECATED. Although it may still work in some browsers, the networkType property was deprecated for
 	// preserving privacy.
-	NetworkType NetworkType `json:"networkType,omitempty"`
+	NetworkType string `json:"networkType,omitempty"`
 
 	// IP is the IP address of the candidate, allowing for IPv4 addresses and
 	// IPv6 addresses, but fully qualified domain names (FQDNs) are not allowed.

stats_go_test.go

@@ -804,7 +804,7 @@ func getStatsSamples() []statSample {
 		Type:          StatsTypeLocalCandidate,
 		ID:            "ILO8S8KYr",
 		TransportID:   "T01",
-		NetworkType:   NetworkTypeUDP4,
+		NetworkType:   "wifi",
 		IP:            "192.168.0.36",
 		Port:          65400,
 		Protocol:      "udp",
@@ -820,7 +820,7 @@ func getStatsSamples() []statSample {
   "type": "local-candidate",
   "id": "ILO8S8KYr",
   "transportId": "T01",
-  "networkType": 1,
+  "networkType": "wifi",
   "ip": "192.168.0.36",
   "port": 65400,
   "protocol": "udp",