Enhance type of headers.get method.

When given a default value that is not None, the get method always
returns a string.

CHANGES.rst

@@ -14,6 +14,7 @@ Unreleased
     decorated with it report the correct type. :issue:`2113`
 -   Fix multipart parsing bug when boundary contains special regex
     characters. :issue:`2125`
+-   Enhance type annotation for ``headers.get``. :issue:`2128`
 
 
 Version 2.0.0

src/werkzeug/datastructures.pyi

@@ -220,6 +220,8 @@ class Headers(Dict[str, str]):
     def __getitem__(self, key: str, _get_mode: Literal[True] = ...) -> str: ...
     def __eq__(self, other: object) -> bool: ...
     @overload  # type: ignore
+    def get(self, key: str, default: str) -> str: ...
+    @overload
     def get(self, key: str, default: Optional[str] = None) -> Optional[str]: ...
     @overload
     def get(

src/werkzeug/sansio/request.py

@@ -491,7 +491,7 @@ def user_agent(self) -> UserAgent:
             Werkzeug 2.1. A ``UserAgent`` subclass must be set to parse
             data from the string.
         """
-        return self.user_agent_class(t.cast(str, self.headers.get("User-Agent", "")))
+        return self.user_agent_class(self.headers.get("User-Agent", ""))
 
     # Authorization