More test code; minor fixes to is.named() and documentation

1) New test code for assert-scalar.R and assertions.R
2) Changed is.named() so it uses `na.rm=TRUE`. See issue #19 
3) Documentation typo fixed
OK, but then `is.named` returns FALSE for the test case in issue #19 (contradicting what we said there). But either way, it does eliminate the possibility of returning NA.