Currently, when a POST request is made using code with build tag
+build !appengine it doesn't set the content-type header passed
to the function. This was breaking the case where a url-encoded
body (a code snippet's body and protocol version) was being POSTed
to golang.org/compile since it never set the content-type of the
request and the corresponding form values parsed out were empty as
a result.

Update golang/go#28080

Change-Id: I677ca01b2f5aecedbd13d1faa7a838ddc0199244
Reviewed-on: https://go-review.googlesource.com/c/140838
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Without changing the behavior of the present command for local
usage (using the local socket for running examples, defaulting to
the current directory for all content). Add flags and set them to
the appropriate values if running on App Engine.

Notably, since the Go files must be in the same directory as
app.yaml, the content root must be ./content/ to avoid listing
the present source files.

It also defaults to running example snippets via the HTTPTransport
(https://play.golang.org/compile) instead of locally when on App
Engine.

There are also some small cleanup code changes.

Update golang/go#28080

Change-Id: I40bb7923107614f88d2bfdffd34a824d4bacb3a1
Reviewed-on: https://go-review.googlesource.com/c/140841
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>