# as_headers errors on invalid types

    Code
      as_headers(1)
    Condition
      Error in `as_headers()`:
      ! `headers` must be a list, character vector, or raw

# has nice print method

    Code
      as_headers(c("X:1", "Y: 2", "Z:"))
    Message
      <httr2_headers>
    Output
      X: 1
      Y: 2
      Z: 
    Code
      as_headers(list())
    Message
      <httr2_headers>

# new_headers checks inputs

    Code
      new_headers(1)
    Condition
      Error in `new_headers()`:
      ! `x` must be a list
    Code
      new_headers(list(1))
    Condition
      Error in `new_headers()`:
      ! All elements of `x` must be named

