Previously when the HaveHTTPStatus() matcher failed, the whole HTTP
response object was printed out, and any message was rendered in
bytes rather than as a string.

This introduces a formatter for HTTP responses, so that the key data is
presented in a helpful format.

Sometimes more than one HTTP status is acceptable. For example a server
may (correctly) return HTTP 204 for an empty response, or it might
return HTTP 200 for an empty response - and we might want to accept
either.

We can now write:
```go
Expect(resp).To(HaveHTTPStatus(http.StatusOK, http.StatusNoContent))
```
And it will match either. This is simpler than using Or() or
SatisfyAny().