Usage of nsIHttpHeaderVisitor Interface

 Call this method to visit all request headers.  Calling setRequestHeader
 while visiting request headers has undefined behavior.  Don't do it!

 @param aVisitor
        the header visitor instance.

void visitRequestHeaders(in nsIHttpHeaderVisitor aVisitor)

 Call this method to visit all response headers.  Calling
 setResponseHeader while visiting response headers has undefined
 behavior.  Don't do it!

 @param aVisitor
        the header visitor instance.

 @throws NS_ERROR_NOT_AVAILABLE if called before the response
         has been received (before onStartRequest).

void visitResponseHeaders(in nsIHttpHeaderVisitor aVisitor)