Don't use braces with standardGeneric()

[krlmlr]

See e.g. difference between dbBind(), which uses braces, and dbConnect(), which doesn't:

> dbBind
nonstandardGenericFunction for "dbBind" defined from package "DBI"

function (res, params, ...) 
{
    standardGeneric("dbBind")
}
<environment: 0x52a27b8>
Methods may be defined for arguments: res, params
Use  showMethods("dbBind")  for currently available ones.
> dbConnect
standardGeneric for "dbConnect" defined from package "DBI"
  defined with value class: "DBIConnection"

function (drv, ...) 
{
    ans <- standardGeneric("dbConnect")
    .valueClassTest(ans, "DBIConnection", "dbConnect")
}
<environment: 0x447acb8>
Methods may be defined for arguments: drv
Use  showMethods("dbConnect")  for currently available ones.

This probably interacts with #135.

Reported by Hervé Pagès.