fix: remove duplicate content types from httpc requests

2 issues from investigating this error. First is the status check func
did not identify it was a media unsupported issue adn tries to unmarshal
the empty response body. The 2nd, was the double content type headers were
causing an error. Locally this error does not surface, cannot repoduce on
macos, but in cloud it is persistent.

closes: #16819

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 1. [16733](https://github.com/influxdata/influxdb/pull/16733): Fix notification rule renaming panics from UI
 1. [16769](https://github.com/influxdata/influxdb/pull/16769): Fix the tooltip for stacked line graphs
+1. [16822](https://github.com/influxdata/influxdb/pull/16822): Fix issue with pkger/http stack crashing on dupe content type
 
 ## v2.0.0-beta.2 [2020-01-24]
 

http/errors.go

@@ -58,6 +58,13 @@ func CheckError(resp *http.Response) (err error) {
 		}
 	}
 
+	if resp.StatusCode == http.StatusUnsupportedMediaType {
+		return &platform.Error{
+			Code: platform.EInvalid,
+			Msg:  fmt.Sprintf("invalid media type: %q", resp.Header.Get("Content-Type")),
+		}
+	}
+
 	contentType := resp.Header.Get("Content-Type")
 	if contentType == "" {
 		// Assume JSON if there is no content-type.
@@ -76,11 +83,9 @@ func CheckError(resp *http.Response) (err error) {
 	switch mediatype {
 	case "application/json":
 		pe := new(platform.Error)
-
-		parseErr := json.Unmarshal(buf.Bytes(), pe)
-		if parseErr != nil {
+		if err := json.Unmarshal(buf.Bytes(), pe); err != nil {
 			line, _ := buf.ReadString('\n')
-			return errors.Wrap(stderrors.New(strings.TrimSuffix(line, "\n")), parseErr.Error())
+			return errors.Wrap(stderrors.New(strings.TrimSuffix(line, "\n")), err.Error())
 		}
 		return pe
 	default:

pkg/httpc/client.go

@@ -152,7 +152,7 @@ func (c *Client) Req(method string, bFn BodyFn, urlPath string, rest ...string)
 		}
 	}
 	if header != "" {
-		headers.Add(header, headerVal)
+		headers.Set(header, headerVal)
 	}
 	// w.Close here is necessary since we have to close any gzip writer
 	// or other writer that requires closing.