[DependencyInjection] Trim constants

| Q             | A
| ------------- | ---
| Bug fix?      | **yes**
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | **yes**
| License       | MIT

When indenting constant parameters in XML configuration files, an exception is throwed.
This PR simply adds:
 * a test for simple constant support in XML configuration files
 * a test for indented constant support in XML configuration files (proving the issue)
 * a fix by trimming constants

## Example to reproduce the bug

Configuration sample:
```XML
<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
  <parameters>
    <parameter key="simple_constant" type="constant">PHP_EOL</parameter>
    <parameter key="indented_constant" type="constant">
        PHP_EOL
    </parameter>
  </parameters>
</container>
```

Code sample:
```PHP
<?php
$container = new ContainerBuilder();

$loader = new XmlFileLoader($container, new FileLocator(__DIR__);
$loader->load('indented_constant.xml');

$container->getParameterBag()->all();
```

Result:
```
constant(): Couldn't find constant 
        PHP_EOL
    
```
Notice the fact that the error message is not:
```
constant(): Couldn't find constant PHP_EOL
```