fix(httpx): Correctly append baggage in async client (#5530)

### Description
Use the same helper function as we do in the sync client. The helper
function makes sure that we avoid duplicating sentry items in the
baggage.

Came up in
#5317 (review)

#### Issues
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* resolves: #1234
* resolves: LIN-1234
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Author: sentrivana

sentry_sdk/integrations/httpx.py

@@ -135,11 +135,8 @@ async def send(
                             key=key, value=value, url=request.url
                         )
                     )
-                    if key == BAGGAGE_HEADER_NAME and request.headers.get(
-                        BAGGAGE_HEADER_NAME
-                    ):
-                        # do not overwrite any existing baggage, just append to it
-                        request.headers[key] += "," + value
+                    if key == BAGGAGE_HEADER_NAME:
+                        add_sentry_baggage_to_headers(request.headers, value)
                     else:
                         request.headers[key] = value