explicit weights=NULL fails in glm,lm (PR#297)

[thomas@biostat.washington.edu]

example(glm)
glm(formula = counts ~ outcome + treatment, family =poisson(),weights=NULL)


This is because model.frame() doesn't treat explicit NULLs the same as
missings. 


version: 
         _                   
platform sparc-sun-solaris2.7
arch     sparc               
os       solaris2.7          
system   sparc, solaris2.7   
status   Patched (unreleased)
major    0                   
minor    65.1                
year     1999                
month    October             
day      07                  
language R                   

and

version:
platform sparc-sun-solaris2.7        
arch     sparc                       
os       solaris2.7                  
system   sparc, solaris2.7           
status   Under development (unstable)
major    0                           
minor    90.0                        
year     1999                        
month    October                     
day      23                          
language R                           



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