Do more validation of headers in mkheaders. Fixes #918.

src/Messages.jl

@@ -200,7 +200,14 @@ resource(uri::URI) = string( isempty(uri.path)     ? "/" :     uri.path,
                             !isempty(uri.fragment) ? "#" : "", uri.fragment)
 
 mkheaders(h::Headers) = h
-mkheaders(h)::Headers = Header[string(k) => string(v) for (k,v) in h]
+function mkheaders(h)::Headers
+    # validation
+    foreach(h) do head
+        head isa String && throw(ArgumentError("header must be passed as key => value pair: `$head`"))
+        length(head) != 2 && throw(ArgumentError("invalid header key-value pair: $head"))
+    end
+    return Header[string(k) => string(v) for (k, v) in h]
+end
 
 method(r::Request) = getfield(r, :method)
 target(r::Request) = getfield(r, :target)

test/messages.jl

@@ -1,7 +1,7 @@
 using Test
 
 using HTTP.Messages
-import HTTP.Messages.appendheader
+import HTTP.Messages: appendheader, mkheaders
 import HTTP.URI
 import HTTP.request
 import HTTP: bytes, nbytes
@@ -17,6 +17,13 @@ using JSON
     http_reads = ["GET", "HEAD", "OPTIONS"]
     http_writes = ["POST", "PUT", "DELETE", "PATCH"]
 
+    @testset "Invalid headers" begin
+        @test_throws ArgumentError mkheaders(["hello"])
+        @test_throws ArgumentError mkheaders([(1,2,3,4,5)])
+        @test_throws ArgumentError mkheaders([1, 2])
+        @test_throws ArgumentError mkheaders(["hello", "world"])
+    end
+
     @testset "Body Length" begin
         @test nbytes(7) === nothing
         @test nbytes(UInt8[1,2,3]) == 3