The current parser recognize "+.inf", "+.Inf" and "+.INF" as a string scalar, but the spec explicitly defines them as part of positive float infinity writings.
To fix the above issue, this PR has corrected the way of scanning scalar contents and converting them into the positive float infinity.
Some test cases have also been added to validate the updated implementation.

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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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Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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