[spector] fix case for payload/xml

[msyyc]

For

typespec/packages/http-specs/specs/payload/xml/main.tsp

Line 332 in afa236a

get(): SimpleModel | XmlError;

,

• SimpleModel is not wrapped with content-type header "application/xml" then its default content-type is "application/json"
• XmlError defines content-type header as "application/xml"

It is very strange that response and error response has such different content-type.

Expected behavor:

We expect SimpleModel shall also be wrapped with content-type "application/xml" to keep consistent with error response like other cases in this file

NOTE:

1. DO add changelog under .chronus/changes
2. DO necessary check before commit.