Fix fsmcontextproxy update (#755)

* Create test case for invalid 'FSMContextProxy.update()' function

* Fix invalid 'FSMContextProxy.update()' function

* Verify that written data has been stored and read back correctly

tests/test_dispatcher/test_fsm_context.py

@@ -0,0 +1,14 @@
+import pytest
+from aiogram.contrib.fsm_storage.memory import MemoryStorage
+from aiogram.dispatcher import FSMContext
+
+
+class TestFSMContext:
+    @pytest.mark.asyncio
+    async def test_update_data(self):
+        context = FSMContext(MemoryStorage(), chat=1, user=1)
+        async with context.proxy() as data:
+            data.update(key1="value1", key2="value2")
+        async with context.proxy() as data:
+            assert data['key1'] == "value1"
+            assert data['key2'] == "value2"