code: public String putCustomRequestHeader(String name, String value) {
          if (customRequestHeaders == null) {
              customRequestHeaders = new HashMap<String, String>();
          }
          return customRequestHeaders.put(name, value);
      }
docs: Put a new custom header to the map of custom header names to custom header values, and return
  the previous value if the header has already been set in this map.
  <p>
  Any custom headers that are defined are used in the HTTP request to the AWS service. These
  headers will be silently ignored in the event that AWS does not recognize them.
  <p>
  NOTE: Custom header values set via this method will overwrite any conflicting values coming
  from the request parameters.
  
  @param name
  The name of the header to add
  @param value
  The value of the header to add
  @return the previous value for the name if it was set, null otherwise