Use POST in Client.fetchImageUrl

Reduces the amount of different request methods we're using.

Not really sure whether this is actually the best option:
- Each request includes `fb_dtsg` and such, so using POST everywhere might be the more secure option?
- But at the same time, the request is more opaque, and harder to debug (urllib3 logs all request urls automatically, so using GET would make that easy)

fbchat/_client.py

@@ -944,8 +944,8 @@ class Client(object):
         """
         image_id = str(image_id)
         data = {"photo_id": str(image_id)}
-        j = self._get(
-            "/mercury/attachments/photo/", query=data, fix_request=True, as_json=True
+        j = self._post(
+            "/mercury/attachments/photo/", data, fix_request=True, as_json=True
         )
 
         url = get_jsmods_require(j, 3)