quoted-strings: Fix explicit octal recognition

PyYAML implements YAML spec version 1.1, not 1.2. Hence, values starting
with `0o` are not considered as numbers: they are just strings, so they
need quotes when `quoted-strings: {required: true}`.

>>> import yaml
>>> yaml.resolver.Resolver().resolve(yaml.nodes.ScalarNode, '100', (True, False))
'tag:yaml.org,2002:int'
>>> yaml.resolver.Resolver().resolve(yaml.nodes.ScalarNode, '0100', (True, False))
'tag:yaml.org,2002:int'
>>> yaml.resolver.Resolver().resolve(yaml.nodes.ScalarNode, '0o100', (True, False))
'tag:yaml.org,2002:str'

Let's try to prevent that.

Fixes https://github.com/adrienverge/yamllint/issues/351.

tests/rules/test_quoted_strings.py

@@ -436,3 +436,21 @@ class QuotedTestCase(RuleTestCase):
                    '- foo bar\n'
                    '- "foo bar"\n',
                    conf, problem1=(3, 3), problem2=(7, 3), problem3=(11, 3))
+
+    def test_octal_values(self):
+        conf = 'quoted-strings: {required: true}\n'
+
+        self.check('---\n'
+                   '- 100\n'
+                   '- 0100\n'
+                   '- 0o100\n'
+                   '- 777\n'
+                   '- 0777\n'
+                   '- 0o777\n'
+                   '- 800\n'
+                   '- 0800\n'
+                   '- 0o800\n'
+                   '- "0800"\n'
+                   '- "0o800"\n',
+                   conf,
+                   problem1=(9, 3), problem2=(10, 3))