Provide a preprocessor that can modify request and response headers

[ghoonch4]

modifyHeaders on Preprocessors can be used to add, set, and remove request or response headers. This can completely replace the removeHeaders on Preprocessors. So if PR is accepted, I think the deprecation of removeHeaders should be considered.

The modifyHeaders can be used like this:

@Test
public void test() throws Exception {
    this.mockMvc.perform(get("/"))
        .andExpect(status().isOk())
        .andDo(document("index"),
            preprocessRequest(
                modifyHeaders()
                    .add("a", "avocado")
                    .set("a", "avocado")
                    .set("a", "avocado", "apple")
                    .remove("a")
                    .remove(Pattern.compile("^a.*"))
                    .remove("a", "avocado")),
            preprocessResponse(
                prettyPrint(),
                modifyHeaders()
                    .add("b", "bravo")));
}

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[ghoonch4]

Hi, @wilkinsona. It's been a long time. I saw 3.0.0-M2 on https://spring.io/projects/spring-restdocs#learn. Is the plan for 2.1.0.RC1 valid?

[wilkinsona]

In an ideal world, there'd still be a 2.1 release that ships with Boot 2.7. Unfortunately, I think I need to be realistic and accept that it's not going to happen due to too many competing commitments. I'll reschedule this for inclusion in 3.0.

[wilkinsona]

Thanks very much, @ghoonch4.