Allow double quotation marks in plain scalars

[fktn-k]

According to the YAML spec, double quotation marks in a plain scalar are allowed. link.
However, fkYAML mistakingly throws an exception against inputs which include such scalars like:

foo: This is "Bar".

So, this PR has fixed the issue and the fix has also been tested by the modified test suite.

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• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
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Please don't

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[coveralls]

coverage: 100.0%. remained the same when pulling 2f589b86a948bc744d0c60779a50f89ff75395ad on bugfix/allow_double_quotes_in_plain_scalars into 4892ae25d3059d85a49c3c12ce17267abdb06fe5 on develop.