Simplify key_builder calling

The keybuilder is either returning a string, or a coroutine or other
awaitable. If the latter, await on the return value to get the string.

fastapi_cache/decorator.py

@@ -116,16 +116,6 @@ def cache(
             key_builder = key_builder or FastAPICache.get_key_builder()
             backend = FastAPICache.get_backend()
 
-            if inspect.iscoroutinefunction(key_builder):
-                cache_key = await key_builder(
-                    func,
-                    namespace,
-                    request=request,
-                    response=response,
-                    args=args,
-                    kwargs=copy_kwargs,
-                )
-            else:
             cache_key = key_builder(
                 func,
                 namespace,
@@ -134,6 +124,9 @@ def cache(
                 args=args,
                 kwargs=copy_kwargs,
             )
+            if inspect.isawaitable(cache_key):
+                cache_key = await cache_key
+
             try:
                 ttl, ret = await backend.get_with_ttl(cache_key)
             except Exception: