Fixed bugs in parsing block scalars

[fktn-k]

In this PR, several bugs in parsing block literal/folded scalars (due to wrong implementations) have been fixed.

1. A leading empty line which is more indented than the first non-empty line

   foo: |
      # this leading line contains 3 spaces as indentation
     foo # 2 spaces as indentation

   • Before: no error was emitted
   • After: parse_error is thrown

2. A non-empty line which is less indented than the content indentation level
   but more indented than the base indentation level

   foo: |
     text
    invalid # this line is less indented than the content indentation level (2)
            # but more indented than the base indentation level (0)

   • Before: no error was emitted
   • After: parse_error is thrown

3. A more-indented empty line in block folded scalar contents

   foo: >
          # 2 spaces(indent) + 1 tab(content) + 1 space(content)
     bar

   • Before: parsed as foo: "\t bar"
   • After: parsed as \n\t \nbar

Furthermore, although the current implementation treats half-width spaces and tabs as indentation characters, only half-width spaces should be treated as the indentation character as the YAML specification defines here.
So, in this PR, detecting an indentation level has been fixed to search only for half-width spaces in parsing block scalars.

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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.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

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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