update forgets about offset() (PR#6656)

In R1.7 and above (including R 1.9 alpha), 'update.formula' forgets to copy any offset(...) term in the original '.' formula:

test> df <- data.frame( x=1:4, y=sqrt( 1:4), z=c(2:4,1))
test> fit1 <- glm( y~offset(x)+z, data=df)
test> fit1$call
glm(formula = y ~ offset(x) + z, data = df)

test> fit1u <- update( fit1, ~.)
test> fit1u$call
glm(formula = y ~ z, data = df)


The problem occurs when 'update.formula' calls 'terms.formula(..., simplify=TRUE)' which defines and calls a function 'fixFormulaObject'. The first line of 'fixFormulaObject' attempts to extract the contents of the RHS of the formula via 

tmp <- attr(terms(object), "term.labels")

but this omits any offsets. Replacing that line with the following,
which I think pulls in everything except the response, *seems* to fix
the problem without disrupting the guts of 'terms' itself:

tmp <- dimnames( attr(terms(object), "factors"))[[1]][ -attr( terms, 'response')]

The suggested line might be simpler than checking the 'offset' component
of 'terms(object)', which won't always exist.

Footnote: strange things happen when there is more than one offset (OK,
there probably shouldn't be, but I thought I'd experiment):

test> fit2 <- glm( y ~ offset( x) + offset( log( x)) + z, data=df)
test> fit2$call
glm(formula = y ~ offset(x) + offset(log(x)) + z, data = df)

test> fit2u <- update( fit2, ~.)
test> fit2u$call
glm(formula = y ~ offset(log(x)) + z, data = df)

Curiously, the 'term.labels' attribute of 'terms(object)' now includes the second offset, but  not the first.