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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• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
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[coveralls]

coverage: 100.0%. remained the same
when pulling 2f589b8 on bugfix/allow_double_quotes_in_plain_scalars
into 4892ae2 on develop.