[#62] Reverse order of GET/POST in HTTP out test

Swap the order of calls to `EmitGET()` and `EmitPOST()` for HTTP output
tests in order to surface the problems:

- GET requests appear to be repeated. For three iterations, 5 GETs and only
  1 POST request can be observed at `StartHTTP()`.
- POST requests are truncated somewhere along the line and cannot be parsed
  by `HTTPOutput.ParseRequest()`. Resulting in:

      Can not parse request POST /pub/WWW/ HTT malformed HTTP version "HTT"

There is an underlying problem with the test that it only checks the request
count, not the type/content of "good" requests.
[#62] HTTPOutput take copy of request byte slice

At the time `CopyMulty()` calls `HTTPOutput.Write(data)` the contents of the
byte slice `data` is correct/consistent. However by the time the goroutine
for `HTTPOutput.sendRequest(data)` is scheduled, the contents of `data` has
changed, which in the case of our tests results in two things happening:

- The same request gets repeated many times.
- A request with the length of `EmitGET()` is made but with the larger
  contents of `EmitPOST()`, causing it to be truncated and
  `HTTPOutput.ParseRequest()` fails.

As I understand it, this is because the slice header of `data` is passed by
value into the goroutine, but the contents referred to by that header are
pointers which subsequently get overwritten.

By taking a `copy()` of the request data into a new buffer variable and
passing that to the goroutine, we can ensure that it doesn't get modified
in-flight.
Merge pull request #65 from alphagov/62-misbehaving_http_output_test

[Fixes #62] HTTPOutput take copy of request byte slice