Add HaveHTTPStatus matcher

[ansd]

Add new HaveHTTPStatus matcher to easily test HTTP status of an HTTP response (recorder).

Inspired by https://relishapp.com/rspec/rspec-rails/v/3-9/docs/matchers/have-http-status-matcher.

Design discussions:

1. Allow ACTUAL to be either a *http.Response or *httptest.ResponseRecorder.
   When unit test-driving a server, a *httptest.ResponseRecorder will probably be used.
   When writing integration tests, a *http.Response will be used.
2. Allow EXPECTED to be an int or a string such that either the StatusCode or Status field can be matched.

A working example demonstrating these different use cases:

package resp_test

import (
	"net/http"
	"net/http/httptest"

	. "github.com/onsi/ginkgo"
	. "github.com/onsi/gomega"
)

var _ = Describe("HaveHTTPStatus", func() {
	It("works on *http.Response", func() {
		resp, err := http.Get("http://www.google.com")

		Expect(err).NotTo(HaveOccurred())
		Expect(resp).To(HaveHTTPStatus(http.StatusOK))
		Expect(resp).To(HaveHTTPStatus("200 OK"))
	})

	It("works on *httptest.ResponseRecorder", func() {
		req, err := http.NewRequest(http.MethodGet, "some URL", nil)
		Expect(err).NotTo(HaveOccurred())

		rr := httptest.NewRecorder()
		h := http.HandlerFunc(someHandler)
		h.ServeHTTP(rr, req)

		Expect(rr).To(HaveHTTPStatus(http.StatusOK))
		Expect(rr).To(HaveHTTPStatus("200 OK"))
	})
})

func someHandler(w http.ResponseWriter, r *http.Request) {
	w.WriteHeader(http.StatusOK)
}