fix unused variable prevent string enforcement

[pubkey]

There was the call const csv = csvString.toString(); which enforces the input of fromString() to be a string. But the variable csv was never used which is why I got the error csvString.substr is not a function deep in a subdependency.

I changed the code to actually use the string enforcement in the later calls.

What do you think about adding a tslint to the CI to prevent these kind of bugs in the future?

[coveralls]

Pull Request Test Coverage Report for Build 235

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 95.421%

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Totals
Change from base Build 234:	0.07%
Covered Lines:	664
Relevant Lines:	680

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💛 - Coveralls

[pubkey]

Maybe it would be better to directly throw if no string was given