Update group_header_rows attribute with new position when adding multiple group_rows to HTML table

There is an issue when adding multiple group_rows to a kable HTML table. When adding the second group_rows statement the positions are wrong because the positions of the first group_header_rows are not updated after adding the new group_header_rows. This line should fix this.

Example .Rmd file:

---
output:
  html_document: default
---

```{r include=FALSE}

# Load libraries
library(tidyverse)
library(kableExtra)

# Create test data
# Nested groups. "a" only contains "1" and "2",
# "b" only contains "3" and "4", "c" only "5" and "d" only "6"
test_data <- tribble(
  ~var1, ~var2, ~var3,
  "a",   "1",    "x",
  "b",   "4",    "y",
  "b",   "3",    "x",
  "a",   "2",    "z",
  "c",   "5",    "y",
  "d",   "6",    "x",
  "d",   "6",    "z",
  "a",   "2",    "y",
  "d",   "7",    "b",
  "d",   "7",    "b")

# Sort data on var1 and var2 so we're able to group rows based on these
test_data_sorted <- test_data %>% 
  arrange(var1, var2)
```

```{r kable_one_group}
# Create kable table and group rows based on var1 first
kable_one_group <- test_data_sorted %>% 
  kable() %>% 
  kable_styling(full_width = FALSE) %>% 
  group_rows(index = test_data_sorted$var1 %>% auto_index(),
             group_label = test_data_sorted$var1)

# This works fine
kable_one_group
```


```{r kable_one_group_other_var}
# Create kable table and group rows based on var2 second
kable_one_group_other_var <- test_data_sorted %>% 
  kable() %>% 
  kable_styling(full_width = FALSE) %>% 
  group_rows(index = test_data_sorted$var2 %>% auto_index(),
             group_label = test_data_sorted$var2)

#This works fine
kable_one_group_other_var
```

```{r kable_two_groups_both_vars}
# Then add another group of rows based on var2 to this table
kable_two_groups <- kable_one_group %>% 
  group_rows(index = test_data_sorted$var2 %>% auto_index(),
             group_label = test_data_sorted$var2 %>% auto_index,
             indent = FALSE)

#This doesn't work as intended
kable_two_groups
```
diff --git a/R/group_rows.R b/R/group_rows.R
index c7d5178..33a517c 100644
--- a/R/group_rows.R
+++ b/R/group_rows.R
@@ -126,6 +126,10 @@
   if (!is.null(group_header_rows)) {
     group_seq <- positions_corrector(group_seq, group_header_rows,
                                      length(xml_children(kable_tbody)))
+    # Update the old group_header_rows attribute with their new positions
+    kable_attrs$group_header_rows <- ifelse(kable_attrs$group_header_rows > group_seq[1],
+                                            kable_attrs$group_header_rows+1,
+                                            kable_attrs$group_header_rows)
   }
 
   # Insert a group header row