code: public Request header(String name, Object value) {
          if (headers == null) {
              headers = new LinkedHashMap<String, Object>();
          }
          headers.put(name, value);
          return this;
      }
docs: Set (or overwrite) an HTTP header value.
  <br>
  Setting a header this way has the highest precedence and overrides a header value set on a {@link Webb}
  instance ({@link Webb#setDefaultHeader(String, Object)}) or a global header
  ({@link Webb#setGlobalHeader(String, Object)}).
  <br>
  Using <code>null</code> or empty String is not allowed for name and value.
  
  @param name name of the header (HTTP-headers are not case-sensitive, but if you want to override your own
  headers, you have to use identical strings for the name. There are some frequently used header
  names as constants in {@link Webb}, see HDR_xxx.
  @param value the value for the header. Following types are supported, all other types use <code>toString</code>
  of the given object:
  <ul>
  <li>{@link java.util.Date} is converted to RFC1123 compliant String</li>
  <li>{@link java.util.Calendar} is converted to RFC1123 compliant String</li>
  </ul>
  @return <code>this</code> for method chaining (fluent API)