check for both stacktraces before calling 'isSameStacktrace'

[zivl]

resolve issue #1125

1. added function in utils.js to check if both given objects are undefined. This function will be called from isSameException for the Exception interface where stacktrace might be missing.
   
   *Note: I've assumed that in such case, where we don't have stacktrace to check, we should return false so Raven will report the exception (even in the price of having duplicates)
2. added test case to simulate the situation

Further more, after investigating issue #1125, I've found the in the function _processException, where the stacktrace object is being built, there is

    var stacktrace;
    if(frames && frames.length){
        stacktrace = {...}
    } 
    else if(fileurl){
        stacktrace = {...}
    }

but no final else, which means that in this case stacktrace resolves to undefined.

I can do some change there, but I don't know what is your exact policy regading such case. So for now I did this extra check, but in my opinion, a proper handling the stacktrace object is better.

[zivl]

@kamilogorek FYI

[kamilogorek]

It makes sense now. I missed isSameException part. Thanks!

[zivl]

@kamilogorek any expectation when is it going to be released?

[kamilogorek]

@zivl I'll release a new version once #1162 gets merged. Hopefully today :)

[kamilogorek]

Released in 3.21.0