Provide a preprocessor that can modify existing headers

[ghoonch4]

I know there is a preprocessor to remove the header. But I could not find a preprocessor that could add or modify headers. How about providing a preprocessor that supports add, set, and remove headers like a modifyParameters on Preprocessors? If you do not mind, I want PR.

For example, it might be useful in the following case:

@Test
public void test() throws Exception {
    this.mockMvc
        .perform(get("/test").header("Authorization", "Basic dXNlcjpzZWNyZXQ="))
        .andExpect(status().isOk())
        .andDo(document("test"));
}

Below is one of the generated .adoc files by the test code.

http-request.adoc

[source,http,options="nowrap"]
----
GET /test HTTP/1.1
Host: localhost:8080
Authorization: Basic dXNlcjpzZWNyZXQ=

----

The value of the Authorization header must be valid to pass the test. But, documents should not expose valid Authorization header value. In this case, I want to modify the value of the Authorization header to an invalid value through preprocessor.

[wilkinsona]

Sounds like a good idea to me. Thanks for the suggestion. A pull request would be most welcome.

[ghoonch4]

I wrote PR #584

[wilkinsona]

Thank you. Closing in favour of PR #584.