hadley/reshape — pull request #38 by tonytonov (2014-01-27 11:34:21+00:00)
Title: Informative message if no id.vars

Sometimes there is a need to melt data "as is", i.e. with no arguments provided:
```r
df <- data.frame(a=1:3, b=4:6)
melt(df)
```
The output is
```r
Using as id variables
variable value
1 a 1
2 a 2
3 a 3
4 b 4
5 b 5
6 b 6
```
The message is not very informative and may be misleading. A small "if" handles this case, changing the message.