Fix tidy_multi_single_dist()

[spsanderson]

Issue 1:
Currently we have:

td <- as.character(.tidy_dist)

Change to:

td <- tolower(as.character(.tidy_dist))

Issue 2:
Currently there is no "tibble_type" in the attributes for the return data.frame. The current attributes are as follows:

  # Attach attributes ----
  attr(df_unnested_tbl, "all") <- atb
  attr(df_unnested_tbl, "tbl") <- "tidy_multi_tibble"
  attr(df_unnested_tbl, ".num_sims") <- max(param_grid$.num_sims)
  attr(df_unnested_tbl, ".param_list") <- .param_list

They need to be:

  # Attach attributes ----
  attr(df_unnested_tbl, "all") <- atb
  attr(df_unnested_tbl, "tibble_type") <- atb$tibble_type
  attr(df_unnested_tbl, "tbl") <- "tidy_multi_tibble"
  attr(df_unnested_tbl, ".num_sims") <- max(param_grid$.num_sims)
  attr(df_unnested_tbl, ".param_list") <- .param_list