Provides access to the map of input headers before a send operation.
Subclasses can modify the headers and then return the same or a different map.
<p>This default implementation in this class returns the input map.
@param headers the headers to send (or {@code null} if none)
@return the actual headers to send (or {@code null} if none)
Provides access to the map of input headers before a send operation. Subclasses can modify the headers and then return the same or a different map. <p>This default implementation in this class returns the input map. @param headers the headers to send (or {@code null} if none) @return the actual headers to send (or {@code null} if none)