fix loadPropsOnServer params

[fraserxu]

Context

This pull request fix "Is the loadPropsOnServer example up-to-date? #63" and related to "#60"

Consideration

The loadPropsOnServer method takes in three params ({ components, params }, loadContext, cb), but from the readme example it only takes two and will cause a cb is not a function error for server side rendering.

// 1. load the props
    loadPropsOnServer(renderProps, (err, asyncProps, scriptTag) => {})

It could be fixed by add an empty loadContext object to the function, but the cleanest way could be to check the type of the loadContext, and this way can make the function work for both 2 and 3 params.

if (typeof loadContext === 'function') {
   cb = loadContext
   loadContext = {}
}