🐛 Fix convert_underscores=False for header Pydantic models (#13515)

docs_src/header_param_models/tutorial003.py

@@ -0,0 +1,19 @@
+from typing import List, Union
+
+from fastapi import FastAPI, Header
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class CommonHeaders(BaseModel):
+    host: str
+    save_data: bool
+    if_modified_since: Union[str, None] = None
+    traceparent: Union[str, None] = None
+    x_tag: List[str] = []
+
+
+@app.get("/items/")
+async def read_items(headers: CommonHeaders = Header(convert_underscores=False)):
+    return headers