Http stream invoke tests (#1231)

* adds parity level of testing http-stream invoke

the other formats had a gamut of tests, now http-stream does too. this makes
obvious some of its behaviors. some things changed / can change now that we
don't have pipes to worry about, the main one being that when containers blow
up now the uds client will get an EOF/ECONNREFUSED instead of the pipe getting
wedged up (allowing us to get the container error easily, previously). I made
my best 50% effort to make a reasonable error for when this happens (similar
to when http/json received garbage errors), open to ideas on verbiage / policy
there.

should be pretty straightforward. one thing to notice is that
http/json/default don't return our fancy new Fn-Http-Status or Fn-Http-H
headers... it's relatively easy to go add this to fdk-go just to test this,
but for invoke I'm really not sure we care (?) and for the gateway, the output
will be identical with the old formats bypassing the header decap. if anybody
has any feelings, feel free to express them.

* fix oomer up for new error

* Adding http header stripping to agent

Adding the header stripping into the agent, this should be low enough
that all routes to fns get treated the same.

api/server/runner_fninvoke.go

@@ -66,6 +66,8 @@ func (s *Server) handleFnInvokeCall2(c *gin.Context) error {
 }
 
 func (s *Server) ServeFnInvoke(c *gin.Context, app *models.App, fn *models.Fn) error {
+	// TODO: we should combine this logic with trigger, which just wraps this block with some headers wizardry
+	// TODO: we should get rid of the buffers, and stream back (saves memory (+splice), faster (splice), allows streaming, don't have to cap resp size)
 	buf := bufPool.Get().(*bytes.Buffer)
 	buf.Reset()
 	writer := syncResponseWriter{
@@ -101,6 +103,7 @@ func (s *Server) ServeFnInvoke(c *gin.Context, app *models.App, fn *models.Fn) e
 	}
 
 	// if they don't set a content-type - detect it
+	// TODO: remove this after removing all the formats (too many tests to scrub til then)
 	if writer.Header().Get("Content-Type") == "" {
 		// see http.DetectContentType, the go server is supposed to do this for us but doesn't appear to?
 		var contentType string