stacktrace -> stackTrace

[barancev]

https://w3c.github.io/webdriver/webdriver-spec.html#processing-model

In SeleniumHQ/selenium#3683 @AutomatedTester wrote:

"We're going to have to update the spec to do stackTrace."

[andreastt]

If the clients are changing anyway to look for stacktrace, why can’t this be a permanent change? I’m skeptical of making API breaking changes at this point because it hits Mozilla, as the first spec conforming implementor, quite hard.

[shs96c]

stacktrace is defined in the spec as an implementation-defined string containing a stack trace report of the active stack frames at the time when the error occurred.

stackTrace from the JSON Wire Protocol is a structured type, composed of a list of objects.

Making the spec use stackTrace will cause unexpected bugs with older selenium clients. I strongly suggest not making the spec change the casing of "stack trace"