[Rd] Allow dot in RHS of update.formula's old formula

[Robert McGehee]

Feature request:

I want to use update.formula to subtract an intercept (or other) term from a formula with a dot on the RHS. However, as this causes an error, I propose a patch below.

Thus, I want:
> update.formula(y ~ ., ~ . -1)
[1] y ~ . - 1

Instead I get this error:
Error in terms.formula(tmp, simplify = TRUE) : 
  '.' in formula and no 'data' argument

While the error message isn't especially helpful (as I *cannot* currently pass in a data argument), the problem is that terms.formula inside update.formula does not allow a dot in the RHS of 'old' unless either a 'data' argument is passed in or 'allowDotAsName=TRUE'. 

Thus, I'd like to suggest this change to update.formula to allow a dot in the RHS of old without (I believe) impacting any other behavior.

-    out <- formula(terms.formula(tmp, simplify = TRUE))
+    out <- formula(terms.formula(tmp, simplify = TRUE, allowDotAsName=TRUE))

If this is undesirable for some reason, then alternatively the dots argument of update.formula could be passed to terms.formula so the user could pass in either 'data' or 'allowDotAsName=TRUE' themselves (though as I cannot think of any reason the user would prefer 'allowDotAsName=FALSE', this is not my preference).

-    out <- formula(terms.formula(tmp, simplify = TRUE))
+    out <- formula(terms.formula(tmp, simplify = TRUE, ...))

>From my reading of the Details section of ?update.formula, it seems as if this suggestion is consistent with the current documentation, as no mention is made of dots in the RHS of 'old', and no mention is made of why this behavior should be otherwise prohibited. If neither change is desirable for some reason, then the update.formula documentation should at least point out this exception (e.g. "... and substitutes the _rhs_ of the 'old' formula for any occurrence of '.' on the right of 'new' *except if there is a dot in the _rhs_ of 'old'*."

Thanks, Robert