[Rd] Reference classes: assignments to fields

[John Chambers]

In R-devel, a recent change (Rev. 56572) makes assignments to fields in 
reference classes consistent with assignments to slots in S4 classes, 
when the field was declared with a class in the call to setRefClass().

The value assigned must come from the declared class for the field, if 
any, or from a subclass of that class.  Previously, if the field had a 
declared class the value for assignment was unconditionally coerced to 
that class.

The added test may produce new error messages, e.g., if you declare a 
field "integer" and assign a numeric, such as 1 rather than 1L.

John