Consider having HeaderWriters check before writing

[jzheaux]

Related to #5193

CacheControlHeaderWriter checks first before writing its corresponding headers.

Since HeaderWriterFilter writes headers at the end of a request, it makes sense that each HeaderWriter implementation would check whether its corresponding headers have already been written before overriding them.

That is, instead of doing:

response.setHeader(name, value)

do:

if (!response.hasHeader(name)) {
    response.setHeader(name, value);
}

[ankurpathak]

@jzheaux I will be working on this.

[ankurpathak]

Here is a pull request:
#6456