code: void parameter(String name, Object value) {
          Preconditions.checkNotNull(name, "parameter name cannot be null");
          if (value instanceof Observable)
              throw new IllegalArgumentException(
                      "use parameters() method not the parameter() method for an Observable");
          this.parameters = parameters.concatWith(Observable.just(new Parameter(name, value)));
      }
docs: Sets a named parameter. If name is null throws a
  {@link NullPointerException}. If value is instance of Observable then
  throws an {@link IllegalArgumentException}.
  
  @param name
  the parameter name. Cannot be null.
  @param value
  the parameter value