Allow `HttpHeaders` invocation on `MockHttpServletRequest` and `MockHttpServletRequestBuilder`

[rwinch]

It's nice that WebClient and RestClient provide access to the rich HttpHeaders object for setting things like HTTP Basic.

WebClient.create().get()
	.uri("/")
	.headers(h -> h.setBasicAuth("user", "password"))
        // ...

It would be nice if MockHttpServletRequest and MockHttpServletRequestBuilder exposed a method that allowed users to leverage HttpHeaders in a similar way as illustrated above.

NOTE: setHeaders(HttpHeaders) is not desirable because it requires setting all of the headers vs updating the existing HttpHeaders.

[rstoyanchev]

We can do this for the MockMvc request builders, but MockHttpServletRequest is more low level and has its own way of storing and formatting headers, so we'll draw the line there.

[rstoyanchev]

See #35588.