fix(sio-client): do not mangle the "_placeholder" attribute

The "_placeholder" attribute is used when sending binary data, and was
incorrectly mangled (converted to a random short property, like "it",
to reduce the bundle size).

This bug was introduced in [1], included in `[email protected]`.

[1]: 7085f0e

Related: #5215

Author: darrachequesne

packages/socket.io-client/support/rollup.config.umd.js

@@ -52,6 +52,7 @@ const prodBundle = {
         mangle: {
           properties: {
             regex: /^_/,
+            reserved: ["_placeholder"],
           },
         },
       }),