Adds a change to handle when it receives stringified json.

[nrodriguez]

Checks if the given field is a stringified JSON object. If it is then it does not change the field.

Fixes: https://github.com/ryu1kn/csv-writer/issues/58

[coveralls]

Pull Request Test Coverage Report for Build 106

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 99.531%

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Totals
Change from base Build 105:	-0.5%
Covered Lines:	153
Relevant Lines:	153

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💛 - Coveralls