bug #15770 [Yaml] Fix the parsing of float keys (jmgq)

This PR was merged into the 2.3 branch.

Discussion
----------

[Yaml] Fix the parsing of float keys

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #15667
| License       | MIT
| Doc PR        |

Before this fix, the parser was trying to set a float as an array key, and according to the [PHP documentation](http://php.net/manual/en/language.types.array.php) *"The key can either be an integer or a string"*. Therefore, PHP was internally casting the key to an integer.

My first approach was to always cast to string, by changing this line:

    $key = Inline::parseScalar($values['key']);

to

    $key = (string) Inline::parseScalar($values['key']);

But that broke some tests, for instance, the parser is expected to transform the key `false` to `0`, but casting `false` to string results in an empty string, rather than `0`.

Commits
-------

520bd26 [Yaml] Fix the parsing of float keys

src/Symfony/Component/Yaml/Parser.php

@@ -133,6 +133,11 @@ class Parser
                     throw $e;
                 }
 
+                // Convert float keys to strings, to avoid being converted to integers by PHP
+                if (is_float($key)) {
+                    $key = (string) $key;
+                }
+
                 if ('<<' === $key) {
                     if (isset($values['value']) && 0 === strpos($values['value'], '*')) {
                         $isInPlace = substr($values['value'], 1);

src/Symfony/Component/Yaml/Tests/ParserTest.php

@@ -685,6 +685,24 @@ bar: 2
 EOF;
         $this->assertEquals(array('foo' => 1, 'bar' => 2), $this->parser->parse($yaml));
     }
+
+    public function testFloatKeys()
+    {
+        $yaml = <<<EOF
+foo:
+    1.2: "bar"
+    1.3: "baz"
+EOF;
+
+        $expected = array(
+            'foo' => array(
+                '1.2' => 'bar',
+                '1.3' => 'baz',
+            ),
+        );
+
+        $this->assertEquals($expected, $this->parser->parse($yaml));
+    }
 }
 
 class B