Fix MockRequest.body type

Commit 0031ff7 changed data from str
to bytes, but forgot to adjust this default value.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk

openapi_core/testing/requests.py

@@ -31,7 +31,7 @@ def __init__(
         self.view_args = view_args
         self.headers = headers
         self.cookies = cookies
-        self.body = data or ""
+        self.body = data or b""
         self.content_type = content_type
 
         self.parameters = RequestParameters(