Handle remote update activities.

test/fixtures/mastodon-update.json

@@ -0,0 +1,43 @@
+{                                                                               
+  "type": "Update",                                                                    
+  "object": {                                                                          
+    "url": "http://mastodon.example.org/@gargron",                                     
+    "type": "Person",                                                                  
+    "summary": "<p>Some bio</p>",                                                              
+    "publicKey": {                                                                     
+      "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA0gs3VnQf6am3R+CeBV4H\nlfI1HZTNRIBHgvFszRZkCERbRgEWMu+P+I6/7GJC5H5jhVQ60z4MmXcyHOGmYMK/\n5XyuHQz7V2Ssu1AxLfRN5Biq1ayb0+DT/E7QxNXDJPqSTnstZ6C7zKH/uAETqg3l\nBonjCQWyds+IYbQYxf5Sp3yhvQ80lMwHML3DaNCMlXWLoOnrOX5/yK5+dedesg2\n/HIvGk+HEt36vm6hoH7bwPuEkgA++ACqwjXRe5Mta7i3eilHxFaF8XIrJFARV0t\nqOu4GID/jG6oA+swIWndGrtR2QRJIt9QIBFfK3HG5M0koZbY1eTqwNFRHFL3xaD\nUQIDAQAB\n-----END PUBLIC KEY-----\n",
+      "owner": "http://mastodon.example.org/users/gargron",
+      "id": "http://mastodon.example.org/users/gargron#main-key"
+    },
+    "preferredUsername": "gargron",
+    "outbox": "http://mastodon.example.org/users/gargron/outbox",
+    "name": "gargle",
+    "manuallyApprovesFollowers": false,
+    "inbox": "http://mastodon.example.org/users/gargron/inbox",
+    "id": "http://mastodon.example.org/users/gargron",
+    "following": "http://mastodon.example.org/users/gargron/following",
+    "followers": "http://mastodon.example.org/users/gargron/followers",
+    "endpoints": {
+      "sharedInbox": "http://mastodon.example.org/inbox"
+    },
+    "icon":{"type":"Image","mediaType":"image/jpeg","url":"https://cd.niu.moe/accounts/avatars/000/033/323/original/fd7f8ae0b3ffedc9.jpeg"},"image":{"type":"Image","mediaType":"image/png","url":"https://cd.niu.moe/accounts/headers/000/033/323/original/850b3448fa5fd477.png"}
+  },
+  "id": "http://mastodon.example.org/users/gargron#updates/1519563538",
+  "actor": "http://mastodon.example.org/users/gargron",
+  "@context": [
+    "https://www.w3.org/ns/activitystreams",
+    "https://w3id.org/security/v1",
+    {
+      "toot": "http://joinmastodon.org/ns#",
+      "sensitive": "as:sensitive",
+      "ostatus": "http://ostatus.org#",
+      "movedTo": "as:movedTo",
+      "manuallyApprovesFollowers": "as:manuallyApprovesFollowers",
+      "inReplyToAtomUri": "ostatus:inReplyToAtomUri",
+      "conversation": "ostatus:conversation",
+      "atomUri": "ostatus:atomUri",
+      "Hashtag": "as:Hashtag",
+      "Emoji": "toot:Emoji"
+    }
+  ]
+}