JSON.stringify is not safe for circular objects

[mrmcduff]

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• I have checked the superset logs for python stacktraces and included it here as text if there are any. (N/A)
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Using the latest Master and the provided sample data, I was getting a repeated crash at https://github.com/apache/incubator-superset/blob/master/superset/assets/src/explore/exploreUtils.js#L74
when creating a Time-series table immediately upon clicking "run query".

The crash in question is a circular-JSON error for JSON.stringify. Drilling down into the JSON in question, it absolutely is circular. Besides fixing this particular data that is being input, we should not use JSON.stringify on anything we aren't 100% sure of the shape of, since this error is relatively easy to create.

Superset version

Master

Expected results

Display Time Series

Actual results

Crash

Steps to reproduce

1. Deploy with Sample data.
2. Create new chart (suggested: use Birth Names)
3. For the time-series column options, choose "Spark Line"

4. Run query. (Click button in top left)
5. Crash.

Stacktrace

Uncaught TypeError: Converting circular structure to JSON
    at JSON.stringify (<anonymous>)
    at getExploreLongUrl (exploreUtils.js?2c11:74)
    at ProxyComponent.addHistory (ExploreViewContainer.jsx?2c1c:237)
    at ProxyComponent.addHistory (react-hot-loader.development.js?c2cb:693)
    at ProxyComponent.onQuery (ExploreViewContainer.jsx?2c1c:162)
    at ProxyComponent.onQuery (react-hot-loader.development.js?c2cb:693)
    at HTMLUnknownElement.callCallback (react-dom.development.js?61bb:149)
    at Object.invokeGuardedCallbackDev (react-dom.development.js?61bb:199)
    at invokeGuardedCallback (react-dom.development.js?61bb:256)
    at invokeGuardedCallbackAndCatchFirstError (react-dom.development.js?61bb:270)

Upon digging in / debugging, the formData element being parsed at that line is absolutely a circular JSON argument.