doc: http response.getHeader() documentation partially incorrect

[mscdex]

• Version: v4.x-master
• Platform: n/a
• Subsystem: doc

The http documentation incorrectly states that response.getHeader() "can only be called before headers get implicitly flushed," which is not true (AFAICT) and we explicitly even test for the ability to safely call response.getHeader() after the headers/response is written. I think this sentence can be removed entirely from the http documentation.