Fix postMessage({}, '*') causes slackin to throw
Fix postMessage({}, '*') causes slackin to throw

Non-slackin code may postMessage() to the same window (in my case, a
Twitter button), and unlike slackin, may choose to send an object as a
message rather than a string. Unlike strings, not all objects have a
.substr() method, so slackin doing e.data.substr() throws an exception:
https://git.io/v2N5t

Ineeza/luka_ineeza_slakin,Venpath/slackin,devopsnz/slackin,mike1pol/slackin,TibiaPF/slackin,thanosIrodotou/slackin,emccode/community,szkl/slackin,edorgeville/slackin-fr,Ineeza/luka_ineeza_slakin,Code-T/slackin,mbbroberg/slackin,IOT-DSA/slackin,TechnologyMasters/technologymasters.herokuapp.com,rauchg/slackin,edorgeville/slackin-fr,Milwaukee-Slack/Milwaukee-Slack-Invite-Website,Code-T/slackin,thanosIrodotou/slackin,mbbroberg/slackin,szkl/slackin,rauchg/slackin,thanosIrodotou/slackin,Milwaukee-Slack/Milwaukee-Slack-Invite-Website,TibiaPF/slackin,IOT-DSA/slackin,avaclancheuk/slackin,mike1pol/slackin,Venpath/slackin,devopsnz/slackin,emccode/community,ouluwebdev/slackin,TechnologyMasters/technologymasters.herokuapp.com,ouluwebdev/slackin,emccode/community,lemonlabsuk/slack-invite-page,avaclancheuk/slackin,lemonlabsuk/slack-invite-page