A patched hackney trace shows this being rejected by the current code in cowboy master:
[hackney trace 40 <0.498.0> 2017:01:28 21:46:36 4833] perform request
Content: [{module,hackney_request},
{line,97},
{header_data,<<"GET /erllambda/erllambda_test HTTP/1.1\r\naccept: application/json\r\nUser-Agent: hackney/1.6.4\r\nHost: %2ftmp%2ferllambda.sock\r\n\r\n">>},
{perform_all,true},
{expect,false}]
[hackney trace 80 <0.498.0> 2017:01:28 21:46:36 4834] got response
Content: [{module,hackney},
{line,372},
{response,
{ok,400,
[{<<"content-length">>,<<"0">>}],
{client,
{1485,639996,831109},
{metrics_ng,metrics_dummy},
hackney_local_tcp,"/tmp/erllambda.sock",0,
<<"/tmp/erllambda.sock:0">>,[],#Port<0.16995>,
{default,#Ref<0.0.2.8596>,
{"/tmp/erllambda.sock",0,hackney_local_tcp},
<0.500.0>,hackney_local_tcp},
#Ref<0.0.2.8596>,true,hackney_pool,5000,false,5,
false,5,nil,nil,
{hparser,response,4096,10,0,on_body,<<>>,
{1,1},
undefined,[],0,undefined,undefined,undefined,
undefined,waiting},
connected,waiting,nil,normal,false,false,false,
undefined,false,#Fun<hackney_request.send.2>,waiting,
nil,4096,<<>>,[],
{1,1},
0,nil,nil,<<"GET">>,<<"/erllambda/erllambda_test">>,
nil}}},
{client,
{client,
{1485,639996,831109},
{metrics_ng,metrics_dummy},
hackney_local_tcp,"/tmp/erllambda.sock",0,
<<"/tmp/erllambda.sock:0">>,[],#Port<0.16995>,
{default,#Ref<0.0.2.8596>,
{"/tmp/erllambda.sock",0,hackney_local_tcp},
<0.500.0>,hackney_local_tcp},
#Ref<0.0.2.8596>,true,hackney_pool,5000,false,5,false,5,
nil,nil,nil,connected,start,nil,normal,false,false,false,
undefined,false,nil,waiting,nil,4096,<<>>,[],undefined,
nil,nil,nil,nil,undefined,nil}}]