Fix bad http GET parameter encoding, add unit test

plugins/inputs/httpjson/httpjson.go

@@ -188,10 +188,10 @@ func (h *HttpJson) sendRequest(serverURL string) (string, float64, error) {
 
 	switch {
 	case h.Method == "GET":
-		requestURL.RawQuery = params.Encode()
 		for k, v := range h.Parameters {
 			params.Add(k, v)
 		}
+		requestURL.RawQuery = params.Encode()
 
 	case h.Method == "POST":
 		requestURL.RawQuery = ""

plugins/inputs/httpjson/httpjson_test.go

@@ -222,7 +222,14 @@ func TestHttpJson200(t *testing.T) {
 
 // Test litecoin sample output
 func TestHttpJsonLiteCoin(t *testing.T) {
+	params := map[string]string{
+		"api_key": "mykey",
+	}
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		err := r.ParseForm()
+		assert.NoError(t, err)
+		key := r.Form.Get("api_key")
+		assert.Equal(t, "mykey", key)
 		w.WriteHeader(http.StatusOK)
 		fmt.Fprintln(w, validJSON2)
 	}))
@@ -232,6 +239,7 @@ func TestHttpJsonLiteCoin(t *testing.T) {
 		Servers:    []string{ts.URL},
 		Name:       "",
 		Method:     "GET",
+		Parameters: params,
 		client:     RealHTTPClient{client: &http.Client{}},
 	}