fix up response headers (#788)

* fix up response headers

* stops defaulting to application/json. this was something awful, go stdlib has
a func to detect content type. sadly, it doesn't contain json, but we can do a
pretty good job by checking for an opening '{'... there are other fish in the
sea, and now we handle them nicely instead of saying it's a json [when it's
not]. a test confirms this, there should be no breakage for any routes
returning a json blob that were relying on us defaulting to this format
(granted that they start with a '{').
* buffers output now to a buffer for all protocol types (default is no longer
left out in the cold). use a little response writer so that we can still let
users write headers from their functions. this is useful for content type
detection instead of having to do it in multiple places.
* plumbs the little content type bit into fn-test-util just so we can test it,
we don't want to put this in the fdk since it's redundant.

I am totally in favor of getting rid of content type from the top level json
blurb. it's redundant, at best, and can have confusing behaviors if a user
uses both the headers and the content_type field (we override with the latter,
now). it's client protocol specific to http to a certain degree, other
protocols may use this concept but have their own way to set it (like http
does in headers..). I realize that it mostly exists because it's somewhat gross
to have to index a list from the headers in certain languages more than
others, but with the ^ behavior, is it really worth it?

closes #782

* reset idle timeouts back

* move json prefix to stack / next to use

api/agent/protocol/http.go

@@ -2,23 +2,15 @@ package protocol
 
 import (
 	"bufio"
-	"bytes"
 	"context"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
-	"strconv"
-	"sync"
 
 	"github.com/fnproject/fn/api/models"
 	opentracing "github.com/opentracing/opentracing-go"
 )
 
-var (
-	bufPool = &sync.Pool{New: func() interface{} { return new(bytes.Buffer) }}
-)
-
 // HTTPProtocol converts stdin/stdout streams into HTTP/1.1 compliant
 // communication. It relies on Content-Length to know when to stop reading from
 // containers stdout. It also mandates valid HTTP headers back and forth, thus
@@ -62,16 +54,6 @@ func (h *HTTPProtocol) Dispatch(ctx context.Context, ci CallInfo, w io.Writer) e
 	span, _ = opentracing.StartSpanFromContext(ctx, "dispatch_http_write_response")
 	defer span.Finish()
 
-	buf := bufPool.Get().(*bytes.Buffer)
-	buf.Reset()
-	defer bufPool.Put(buf)
-
-	// copy the response body into a buffer so that we read the whole thing. then set the content length.
-	io.Copy(buf, resp.Body)
-	resp.Body.Close()
-	resp.Body = ioutil.NopCloser(buf)
-	resp.Header.Set("Content-Length", strconv.Itoa(buf.Len()))
-
 	rw, ok := w.(http.ResponseWriter)
 	if !ok {
 		// async / [some] tests go through here. write a full http request to the writer