Make functions type consistent

[richierocks]

Some functions sometimes but not always convert their result to be a data frame, which make them difficult to program against and to reason about.

For example asn_tasks_find_by_project() will return a tibble if there are tasks in the project, but returns a list of class asana_api otherwise.

To reproduce,

library(asana)
library(dcasana)
library(dplyr)

team_content_quality_gid <- dc_get_teams() %>%
  filter(name == "Content Quality") %>%
  pull(gid)

test_project <- asn_projects_create_in_team(
  team = team_content_quality_gid,
  name = "test-autogenerated-project",
  notes = "This is a test. Please delete this project."
)
# There are no tasks in the project, so the return value is a list
test_project_gid <- test_project$content$data$gid
asn_tasks_find_by_project(project_id = test_project_gid)
#> <Asana /projects/1127053277614384/tasks>
#> List of 1
#>  $ data: list()

# Once you add a task, you get a tibble
test_task <- asn_tasks_create(
  projects = test_project$content$data$gid,
  name = 'Test Task 1'
)
asn_tasks_find_by_project(project_id = test_project_gid)
#> # A tibble: 1 x 4
#>   id               gid              name        resource_type
#>   <chr>            <chr>            <chr>       <chr>        
#> 1 1127053162816649 1127053162816649 Test Task 1 task