is2 gives weird failure msg for vectorized classes arg

[Richard Cotton]

assertive.base::is2(1:5, c("call", "formula", "character"))
## There were 3 failures:
##   Position     Value           Cause
## 1        1      call class 'integer'
## 2        2   formula class 'integer'
## 3        3 character class 'integer'

[Richard Cotton]

better feedback msg in https://bitbucket.org/richierocks/assertive.base/commits/b69e3c63f29fdab69bda2cc5a9e49f135672074a