update mus.js

[danklee19]

Setting custom data don't work, because JSON.parse is missing...added

[mauriciogior]

@danklee19 Thanks. Could you add a check to verify if data is a JSON or a String prior to using JSON.parse?

[danklee19]

Ok, there are one simple check (first and last character)
The second check (regex) is not fine or complete, but it works 😄