Cleanup header initialization to avoid excessive copies

Set keepalive to true by default for tcp layer.
Fix bug where both StatusCodes & Messages were exporting `statustext`.

src/ConnectionPool.jl

@@ -410,7 +410,7 @@ end
 function getconnection(::Type{TCPSocket},
                        host::AbstractString,
                        port::AbstractString;
-                       keepalive::Bool=false,
+                       keepalive::Bool=true,
                        connect_timeout::Int=0,
                        readtimeout::Int=0,
                        kw...)::TCPSocket

src/HTTP.jl

@@ -82,7 +82,10 @@ println(String(r.body))
 `[string(k) => string(v) for (k,v) in headers]` yields `Vector{Pair}`.
 e.g. a `Dict()`, a `Vector{Tuple}`, a `Vector{Pair}` or an iterator.
 By convention, if a header _value_ is an empty string, it will not be written
-when sending a request (following the curl convention).
+when sending a request (following the curl convention). By default, a copy of
+provided headers is made (since required headers are typically set during the request);
+to avoid this copy and have HTTP.jl mutate the provided headers array, pass `copyheaders=false`
+as an additional keyword argument to the request.
 
 `body` can be a variety of objects:
 
@@ -278,7 +281,7 @@ HTTP.open("POST", "http://music.com/play") do io
 end
 ```
 """
-function request(method, url, h=Header[], b=nobody;
+function request(method, url, h=nothing, b=nobody;
                  headers=h, body=b, query=nothing, kw...)::Response
     return request(HTTP.stack(), method, url, headers, body, query; kw...)
 end
@@ -418,9 +421,9 @@ function stack(
     return messagelayer(debuglayer(layers4))
 end
 
-function request(stack::Base.Callable, method, url, h=Header[], b=nobody, q=nothing;
+function request(stack::Base.Callable, method, url, h=nothing, b=nobody, q=nothing;
                  headers=h, body=b, query=q, kw...)::Response
-    return stack(string(method), request_uri(url, query), mkheaders(headers), body; kw...)
+    return stack(string(method), request_uri(url, query), headers, body; kw...)
 end
 
 macro remove_linenums!(expr)

src/Messages.jl

@@ -57,7 +57,7 @@ export Message, Request, Response,
        readchunksize,
        writeheaders, writestartline,
        bodylength, unknown_length,
-       payload, decode, statustext, sprintcompact
+       payload, decode, sprintcompact
 
 using URIs, CodecZlib
 using ..Pairs, ..IOExtras, ..Parsers, ..Strings, ..Forms, ..Conditions
@@ -204,13 +204,14 @@ resource(uri::URI) = string( isempty(uri.path)     ? "/" :     uri.path,
                             !isempty(uri.fragment) ? "#" : "", uri.fragment)
 
 mkheaders(h::Headers) = h
-function mkheaders(h)::Headers
+function mkheaders(h, headers=Vector{Header}(undef, length(h)))::Headers
     # validation
-    foreach(h) do head
+    for (i, head) in enumerate(h)
         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"))
+        headers[i] = SubString(string(head[1])) => SubString(string(head[2]))
     end
-    return Header[string(k) => string(v) for (k, v) in h]
+    return headers
 end
 
 method(r::Request) = getfield(r, :method)

src/clientlayers/MessageRequest.jl

@@ -7,15 +7,21 @@ using ..Strings: HTTPVersion
 
 export messagelayer
 
+# like Messages.mkheaders, but we want to make a copy of user-provided headers
+# and avoid double copy when no headers provided (very common)
+mkreqheaders(::Nothing, ch) = Header[]
+mkreqheaders(headers::Headers, ch) = ch ? copy(headers) : headers
+mkreqheaders(h, ch) = mkheaders(h)
+
 """
     messagelayer(handler) -> handler
 
 Construct a [`Request`](@ref) object from method, url, headers, and body.
 Hard-coded as the first layer in the request pipeline.
 """
 function messagelayer(handler)
-    return function(method::String, url::URI, headers::Headers, body; response_stream=nothing, http_version=HTTPVersion(1, 1), kw...)
-        req = Request(method, resource(url), headers, body; url=url, version=http_version, responsebody=response_stream)
+    return function(method::String, url::URI, headers, body; copyheaders::Bool=true, response_stream=nothing, http_version=HTTPVersion(1, 1), kw...)
+        req = Request(method, resource(url), mkreqheaders(headers, copyheaders), body; url=url, version=http_version, responsebody=response_stream)
         local resp
         try
             resp = handler(req; response_stream=response_stream, kw...)