Prior to this commit, the `ServletServerHttpResponse` would copy headers
from the `HttpHeaders` map and calls methods related to headers exposed
as properties (content-type, character encoding).
Unlike its reactive variant, this would not set the content length.
Depending on the Servlet container implementation, this could cause
duplicate Content-Length response headers in the actual HTTP response.
This commit aligns both implementations and ensures that the
`setContentLengthLong` method is called if necessary so that the Servlet
container can ensure a single header for that.
Fixes gh-26330
dcc8dcdff8