Fix: methods cannot be used as constructor

[andreabergia]

The spec says that invoking a method as a constructor is not valid, but invoking normal functions is. This is, for example, node:

> o = { method() {}, notAMethod: function() {} }
{ method: [Function: method], notAMethod: [Function: notAMethod] }
> new o.method()
Uncaught TypeError: o.method is not a constructor
> new o.notAMethod()
notAMethod {}

This PR implements the same behavior in Rhino, and fixes #1299.

[gbrail]

This makes sense to me, but I wonder how much existing code it could break and whether we should put it behind a "VERSION_ES6" or a "VERSION_ECMASCRIPT" flag just in case -- then again, since it didn't break any of MozillaSuiteTest perhaps it's not a big problem.

[gbrail]

I'm willing to be convinced on this, but I think that since this breaks things that may have worked before, we should check that language version is >= VERSION_ES6 before throwing this exception. I'm not sure that we will put this in 1.8.0, but can merge it soon after.

[andreabergia]

I'm willing to be convinced on this, but I think that since this breaks things that may have worked before, we should check that language version is >= VERSION_ES6 before throwing this exception.

Rebased onto master and added a check on the language version.