code: @Deprecated
      public ProxyHandler addRequestHeader(final HttpString header, final String value) {
          requestHeaders.put(header, ExchangeAttributes.constant(value));
          return this;
      }
docs: Adds a request header to the outgoing request. If the header resolves to null or an empty string
  it will not be added, however any existing header with the same name will be removed.
  
  @param header The header name
  @param value  The header value attribute.
  @return this