Missing expression in Pipe closures should preserve Pipe object

[renkun-ken]

If expr is missing in .(expr) or I(expr) in Pipe object in calling, it should be interpreted as doing nothing for conceptual consistent.

A useful case is that if one assign the closure to other symbols, directly calling the function without argument should show the current Pipe contents.

> dot <- Pipe(mtcars)$lm(formula = mpg ~ wt + cyl)$.
> dot()
$value : lm 
------

Call:
lm(formula = mpg ~ wt + cyl, data = .)

Coefficients:
(Intercept)           wt          cyl  
     39.686       -3.191       -1.508  

> dot(coefficients)
$value : numeric 
------
(Intercept)          wt         cyl 
  39.686261   -3.190972   -1.507795