code: public SingleNodeAssert hasAttribute(String attributeName, String attributeValue) {
          isNotNull();
  
          final Map.Entry<QName, String> attribute = attributeForName(attributeName);
          if (attribute == null || !attribute.getValue().equals(attributeValue)) {
              throwAssertionError(shouldHaveAttributeWithValue(actual.getNodeName(), attributeName, attributeValue));
          }
  
          return this;
      }
docs: Verifies that node has attribute with given name and value.
  
  @throws AssertionError if the actual node is {@code null}.
  @throws AssertionError if node has not attribute with given name and value.