Add TopLayer layer at the top of every stack

[christopher-dG]

This is a fix for JuliaTesting/BrokenRecord.jl#20.

BrokenRecord adds an extra default layer that is meant to go at the top of the stack, so the before layer is top_layer(stack()). Currently, that's RedirectLayer. But afterwards, if you try to create a stack with redirects disabled (AWS.jl does this), HTTP can't insert the extra layer anywhere.

This introduces a TopLayer that is always at the top of stack(), and does nothing but pass on its arguments to the next layer. That way, we can guarantee that a layer inserted before top_layer(stack()) will always be insertable.

Before:

julia> using HTTP

julia> abstract type MyLayer{Next <: HTTP.Layer} <: HTTP.Layer{Next} end

julia> HTTP.insert_default!(HTTP.top_layer(HTTP.stack()), MyLayer);

julia> HTTP.stack()
MyLayer{RedirectLayer{BasicAuthLayer{MessageLayer{RetryLayer{ExceptionLayer{ConnectionPoolLayer{StreamLayer{Union{}}}}}}}}}

julia> HTTP.stack(; redirect=false)
ERROR: HTTP.Layers.LayerNotFoundException: RedirectLayer not found in BasicAuthLayer{MessageLayer{RetryLayer{ExceptionLayer{ConnectionPoolLayer{StreamLayer{Union{}}}}}}}

Stacktrace:
  [1] insert(stack::Type{BasicAuthLayer{MessageLayer{RetryLayer{ExceptionLayer{ConnectionPoolLayer{StreamLayer{Union{}}}}}}}}, layer_before::Type{RedirectLayer}, custom_layer::Type{MyLayer})
    @ HTTP.Layers ~/Code/HTTP/src/layers.jl:110
  [2] (::HTTP.var"#24#25")(stack::Type, ::Tuple{UnionAll, UnionAll})
    @ HTTP ~/Code/HTTP/src/HTTP.jl:603
  [3] BottomRF
    @ ./reduce.jl:81 [inlined]
  [4] _foldl_impl(op::Base.BottomRF{HTTP.var"#24#25"}, init::Type, itr::Set{Tuple{Union{Type{Union{}}, UnionAll}, UnionAll}})
    @ Base ./reduce.jl:58
  [5] foldl_impl
    @ ./reduce.jl:48 [inlined]
  [6] mapfoldl_impl(f::typeof(identity), op::HTTP.var"#24#25", nt::Type, itr::Set{Tuple{Union{Type{Union{}}, UnionAll}, UnionAll}})
    @ Base ./reduce.jl:44
  [7] mapfoldl(f::Function, op::Function, itr::Set{Tuple{Union{Type{Union{}}, UnionAll}, UnionAll}}; init::Type)
    @ Base ./reduce.jl:160
  [8] #mapreduce#218
    @ ./reduce.jl:287 [inlined]
  [9] #reduce#220
    @ ./reduce.jl:456 [inlined]
 [10] stack(; redirect::Bool, aws_authorization::Bool, cookies::Bool, canonicalize_headers::Bool, retry::Bool, status_exception::Bool, readtimeout::Int64, detect_content_type::Bool, verbose::Int64, kw::Base.Iterators.Pairs{Union{}, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
    @ HTTP ~/Code/HTTP/src/HTTP.jl:602
 [11] top-level scope
    @ REPL[5]:1

[codecov-commenter]

Codecov Report

Merging #737 (04c3832) into master (ed987aa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #737   +/-   ##
=======================================
  Coverage   76.88%   76.89%           
=======================================
  Files          37       38    +1     
  Lines        2444     2445    +1     
=======================================
+ Hits         1879     1880    +1     
  Misses        565      565           

Impacted Files	Coverage Δ
src/HTTP.jl	97.82% <100.00%> (ø)
src/TopRequest.jl	100.00% <100.00%> (ø)

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[quinnj]

LGTM