Don't use omitempty for ICECandidateInit

Always return sdpMid & sdpMLineindex value if it
is null. The W3C starts every attribute definition with
`If not null ...`

icecandidateinit.go

@@ -3,7 +3,7 @@ package webrtc
 // ICECandidateInit is used to serialize ice candidates
 type ICECandidateInit struct {
 	Candidate        string  `json:"candidate"`
-	SDPMid           *string `json:"sdpMid,omitempty"`
-	SDPMLineIndex    *uint16 `json:"sdpMLineIndex,omitempty"`
-	UsernameFragment string  `json:"usernameFragment"`
+	SDPMid           *string `json:"sdpMid"`
+	SDPMLineIndex    *uint16 `json:"sdpMLineIndex"`
+	UsernameFragment *string `json:"usernameFragment"`
 }