Fix HTTP header content-length when using compression (prometheus#413)

Signed-off-by: Calu <lucas.boisserie@gmail.com>

Author: LucasBoisserie

simpleclient_httpserver/src/main/java/io/prometheus/client/exporter/HTTPServer.java

@@ -65,15 +65,15 @@ public void handle(HttpExchange t) throws IOException {
 
             t.getResponseHeaders().set("Content-Type",
                     TextFormat.CONTENT_TYPE_004);
-            t.getResponseHeaders().set("Content-Length",
-                    String.valueOf(response.size()));
             if (shouldUseCompression(t)) {
                 t.getResponseHeaders().set("Content-Encoding", "gzip");
                 t.sendResponseHeaders(HttpURLConnection.HTTP_OK, 0);
                 final GZIPOutputStream os = new GZIPOutputStream(t.getResponseBody());
                 response.writeTo(os);
                 os.finish();
             } else {
+                t.getResponseHeaders().set("Content-Length",
+                        String.valueOf(response.size()));
                 t.sendResponseHeaders(HttpURLConnection.HTTP_OK, response.size());
                 response.writeTo(t.getResponseBody());
             }