tracker issue : CF-4205245

Problem Description:

When a fat arrow function has curly braces around the body, ONLY an explicit return statement should return a value.  The implicit return behavior only applies to a function body with no curlies. 

This is following the standard JavaScript implementation of fat arrow functions:
https://stackoverflow.com/questions/35440265/curly-brackets-in-arrow-functions

Please fix this quickly as it will be a breaking change since fat arrows are already out now in update 5.

Steps to Reproduce:

{code:java}
<cfscript>
// Incorrectly returns 4.  Should be undefined since no explicit return statement is used inside of curlies
func = x =>  { x+1 }
writeDump( func(3) )
</cfscript>
{code}

Actual Result:

4

Expected Result:

undefined

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