Use method as cache key prefix for non-GET requests. (#75)

The response for a HEAD request shouldn't be served in response to a
GET request.

In practice, HEAD responses contain http.noBody values, which return
empty body if you try to read them. Prior to this change, that's what
happened if you hit a server with a HEAD request and then GET the same
URL. The GET response would be incorrectly served from cache with an
empty body (that of the HEAD response).

Rather than prefix all keys, we add prefixing to non-GET requests only.
As well as saving the cost of a string concatenation, it ensures
existing disk caches can upgrade without completely invalidating.

httpcache.go

@@ -41,7 +41,11 @@ type Cache interface {
 
 // cacheKey returns the cache key for req.
 func cacheKey(req *http.Request) string {
-	return req.URL.String()
+	if req.Method == http.MethodGet {
+		return req.URL.String()
+	} else {
+		return req.Method + " " + req.URL.String()
+	}
 }
 
 // CachedResponse returns the cached http.Response for req if present, and nil

httpcache_test.go

@@ -218,6 +218,30 @@ func TestCacheableMethod(t *testing.T) {
 	}
 }
 
+func TestDontServeHeadResponseToGetRequest(t *testing.T) {
+	resetTest()
+	url := s.server.URL + "/"
+	req, err := http.NewRequest(http.MethodHead, url, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = s.client.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req, err = http.NewRequest(http.MethodGet, url, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp, err := s.client.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.Header.Get(XFromCache) != "" {
+		t.Errorf("Cache should not match")
+	}
+}
+
 func TestDontStorePartialRangeInCache(t *testing.T) {
 	resetTest()
 	{