Adding defaults for RTCRtpEncodingParameters.active and priority

Fixes #1491.

The rest of the encoding parameters (`ptime`, `maxBitrate`,
`maxFramerate`, etc.) are optional, but `active` and `priority` need
default values in case they aren't specified in the `sendEncodings`
passed into `addTransceiver`.

webrtc.html

@@ -6289,8 +6289,8 @@ <h2>Dictionary <dfn>RTCRtpParameters</dfn> Members</h2>
              RTCRtpRtxParameters rtx;
              RTCRtpFecParameters fec;
              RTCDtxStatus        dtx;
-             boolean             active;
-             RTCPriorityType     priority;
+             boolean             active = true;
+             RTCPriorityType     priority = "low";
              unsigned long       ptime;
              unsigned long       maxBitrate;
              double              maxFramerate;
@@ -6337,7 +6337,8 @@ <h2>Dictionary <dfn>RTCRtpEncodingParameters</dfn>
               sender.</p>
             </dd>
             <dt><dfn><code>active</code></dfn> of type <span class=
-            "idlMemberType"><a>boolean</a></span></dt>
+            "idlMemberType"><a>boolean</a></span>, defaulting to
+            <code>true</code></dt>
             <dd>
               <p>For an <code><a>RTCRtpSender</a></code>, indicates that this
               encoding is actively being sent. Setting it to <code>false</code>
@@ -6348,7 +6349,8 @@ <h2>Dictionary <dfn>RTCRtpEncodingParameters</dfn>
               decoded.</p>
             </dd>
             <dt><dfn><code>priority</code></dfn> of type <span class=
-            "idlMemberType"><a>RTCPriorityType</a></span></dt>
+            "idlMemberType"><a>RTCPriorityType</a></span>, defaulting to
+            <code>"low"</code></dt>
             <dd>
               <p>Indicates the priority of this encoding. It is specified in
               [[!RTCWEB-TRANSPORT]], Section 4.</p>