table_mode_align_char doesn't work with multi-line headers

Suppose I make a table with a right alignment marker in the second column:
```
| first thing | second thing |
|-------------+-------------:|
| 1 | 2 |
```
If I run `:TableModeRealign`, the second column is right-aligned as expected:
```
| first thing | second thing |
|-------------+-------------:|
| 1 | 2 |
```
But if I use multiple lines in the header:
```
| first | second |
| thing | thing |
|-------+-------:|
| 1 | 2 |
```
then run `:TableModeRealign`, I instead get left alignment:
```
| first | second |
| thing | thing |
|-------+-------:|
| 1 | 2 |
```
This seems to be caused by the interaction of `tablemode#align#Align` and `tablemode#align#alignments`, together assuming a border line will appear after the first text line.