Spring MVC Test has a "defaultRequest" feature in the MockMvc setup. I suppose this request here is in part inspired by that? Or at least is in comparison to that feature which is based on the same RequestBuilder for defining default and regular request, along with a merge of the two builder instances checking first if a request property has been set before merging it.
We now support the concept of a "base URI" in what is currently WebClientOperations. We could also allow declaring default headers and/or cookies to go with that base URI: