code: @Deprecated
      public ProxyHandler addRequestHeader(final HttpString header, final String attribute, final ClassLoader classLoader) {
          requestHeaders.put(header, ExchangeAttributes.parser(classLoader).parse(attribute));
          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.
  <p>
  The attribute value will be parsed, and the resulting exchange attribute will be used to create the actual header
  value.
  
  @param header    The header name
  @param attribute The header value attribute.
  @return this