Fix three shadow variable warnings (govet -shadow)

Fixes:
http.go:118: declaration of "part" shadows declaration at http.go:117
http_test.go:50: declaration of "respBody" shadows declaration at http_test.go:25
promhttp/http.go:305: declaration of "part" shadows declaration at promhttp/http.go:304

Signed-off-by: Karsten Weiss <knweiss@gmail.com>

prometheus/http.go

@@ -115,7 +115,7 @@ func decorateWriter(request *http.Request, writer io.Writer) (io.Writer, string)
 	header := request.Header.Get(acceptEncodingHeader)
 	parts := strings.Split(header, ",")
 	for _, part := range parts {
-		part := strings.TrimSpace(part)
+		part = strings.TrimSpace(part)
 		if part == "gzip" || strings.HasPrefix(part, "gzip;") {
 			return gzip.NewWriter(writer), "gzip"
 		}

prometheus/http_test.go

@@ -47,9 +47,9 @@ func TestInstrumentHandler(t *testing.T) {
 	instant := time.Now()
 	end := instant.Add(30 * time.Second)
 	now = nowSeries(instant, end)
-	respBody := respBody("Howdy there!")
+	body := respBody("Howdy there!")
 
-	hndlr := InstrumentHandler("test-handler", respBody)
+	hndlr := InstrumentHandler("test-handler", body)
 
 	opts := SummaryOpts{
 		Subsystem:   "http",

prometheus/promhttp/http.go

@@ -302,7 +302,7 @@ func decorateWriter(request *http.Request, writer io.Writer, compressionDisabled
 	header := request.Header.Get(acceptEncodingHeader)
 	parts := strings.Split(header, ",")
 	for _, part := range parts {
-		part := strings.TrimSpace(part)
+		part = strings.TrimSpace(part)
 		if part == "gzip" || strings.HasPrefix(part, "gzip;") {
 			return gzip.NewWriter(writer), "gzip"
 		}