Update in_memory.py to fix "TypeError: keywords must be strings" (#7202)

Update in_memory.py to fix "TypeError: keywords must be strings" on
certain dictionaries

Simple fix to prevent a "TypeError: keywords must be strings" error I
encountered in my use case.

@baskaryan 

Thanks! Hope useful!

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

langchain/docstore/in_memory.py

@@ -17,7 +17,7 @@ class InMemoryDocstore(Docstore, AddableMixin):
         overlapping = set(texts).intersection(self._dict)
         if overlapping:
             raise ValueError(f"Tried to add ids that already exist: {overlapping}")
-        self._dict = dict(self._dict, **texts)
+        self._dict = {**self._dict, **texts}
 
     def search(self, search: str) -> Union[str, Document]:
         """Search via direct lookup."""