docs: public final Jar setAttribute(String section, String name, String value) {
          verifyNotSealed();
          if (jos != null)
              throw new IllegalStateException("Manifest cannot be modified after entries are added.");
          Attributes attr = getManifest().getAttributes(section);
          if (attr == null) {
              attr = new Attributes();
              getManifest().getEntries().put(section, attr);
          }
          attr.putValue(name, value);
          return this;
      }