docs: public void setBoundary(byte[] boundary)
              throws IllegalBoundaryException {
          if (boundary.length != boundaryLength - BOUNDARY_PREFIX.length) {
              throw new IllegalBoundaryException(
                      "The length of a boundary token can not be changed");
          }
          System.arraycopy(boundary, 0, this.boundary, BOUNDARY_PREFIX.length,
                  boundary.length);
      }