Fix na colname

Hi,

In the current `knitr` version, applying `kable` to a table or matrix with missing colname generates an error : 

```r
> m <- matrix(c(1, 1, 1, 1), ncol = 2, dimnames = list(c('A', NA), c('B', NA)))
> m
     B <NA>
A    1    1
<NA> 1    1
> kable(m)
Erreur dans rep.int(string[i], times[i]) : valeur 'times' incorrecte
```

This pull request fixes this error by replacing any missing column name with the `"NA"` character string. Then `kable(m)` gives : 

```r
> kable(m)


|   |  B| NA|
|:--|--:|--:|
|A  |  1|  1|
|NA |  1|  1|
```