Fix Event default timestamp (#24)

Co-authored-by: calle <93376500+callebtc@users.noreply.github.com>

nostr/event.py

@@ -19,7 +19,7 @@ class Event():
             self, 
             public_key: str, 
             content: str, 
-            created_at: int=int(time.time()), 
+            created_at: int = None, 
             kind: int=EventKind.TEXT_NOTE, 
             tags: "list[list[str]]"=[], 
             id: str=None, 
@@ -30,7 +30,7 @@ class Event():
         self.id = id if not id is None else Event.compute_id(public_key, created_at, kind, tags, content)
         self.public_key = public_key
         self.content = content
-        self.created_at = created_at
+        self.created_at = created_at or int(time.time())
         self.kind = kind
         self.tags = tags
         self.signature = signature