warmup draws omitted from stanfit objects

Right now, the conversion of stanfit objects into the draws format relies on the as.array function from Rstan. It would be great to document this somewhere and I would actually expect the function to convert all draws in the stanfit object, but the function drops the warmup samples without a way to influence this as the following example demonstrates:

library(rstan)
library(posterior)

ex_model_code <- '
  parameters {
    real alpha[2,3];
    real beta[2]; 
  } 
  model {
    for (i in 1:2) for (j in 1:3) 
      alpha[i, j] ~ normal(0, 1); 
    for (i in 1:2) 
      beta ~ normal(0, 2); 
  } 
'

## fit the model 
fit <- stan(model_code = ex_model_code, chains = 4) 

## we sampled and kept the warmup samples
fit

draws_post <- as_draws(fit)

## but converting to draws will omit the warmup draws! There is no way
## to get the warmup draws, since "as.array" is called. The as.array
## call will by default use "extract(x, permuted = FALSE, inc_warmup =
## FALSE)" such that warmup is always dropped.
draws_post

## I think, the default should be to return the warmup draws like so:
draws_post_alt <- as_draws(rstan::extract(fit, permuted = FALSE, inc_warmup = TRUE))

draws_post_alt

Start by tracing as_draws(fit) through the RStan as.array conversion and compare it with rstan::extract(..., inc_warmup = TRUE) in the issue example. Check how warmup draws are retained in a stanfit object and where conversion behavior is documented. Done means the conversion can include warmup draws as requested and the behavior or option is documented.