doc: reword message.headers

message.headers states that the headers are read-only, when in fact they
are not. This change rewords the docs to a more appropriate description,
while not promoting this kind of behavior.

Issue: nodejs#3146

doc/api/http.markdown

@@ -806,7 +806,7 @@ Just like `'end'`, this event occurs only once per response.
 
 The request/response headers object.
 
-Read only map of header names and values. Header names are lower-cased.
+Key-value pairs of header names and values. Header names are lower-cased.
 Example:
 
     // Prints something like: