Space as a delimiter tests

https://github.com/dantleech/symfony-form-array-to-delimited-string-transformer/issues/5

Tests result:

```
$ vendor/bin/phpunit
PHPUnit 4.3.1 by Sebastian Bergmann.

Configuration read from symfony-form-array-to-delimited-string-transformer/phpunit.xml.dist

...............FFFF

Time: 85 ms, Memory: 3.75Mb

There were 4 failures:

1) DTL\Symfony\Form\DataTransformer\ArrayToDelimitedStringTransformerTest::testSpaceAsDelimiter with data set #2 (array('', 'foo'), ' foo', 'Empty elements at the beginning are preserved')
Empty elements at the beginning are preserved
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
- foo
+foo

symfony-form-array-to-delimited-string-transformer/tests/ArrayToDelimitedStringTransformerTest.php:132

2) DTL\Symfony\Form\DataTransformer\ArrayToDelimitedStringTransformerTest::testSpaceAsDelimiter with data set #3 (array('foo', ''), 'foo ', 'Empty elements at the end are preserved')
Empty elements at the end are preserved
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-foo 
+foo

symfony-form-array-to-delimited-string-transformer/tests/ArrayToDelimitedStringTransformerTest.php:132

3) DTL\Symfony\Form\DataTransformer\ArrayToDelimitedStringTransformerTest::testSpaceAsDelimiter with data set #4 (array('', '', 'foo'), '  foo', 'Empty elements at the beginning are preserved')
Empty elements at the beginning are preserved
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-  foo
+foo

symfony-form-array-to-delimited-string-transformer/tests/ArrayToDelimitedStringTransformerTest.php:132

4) DTL\Symfony\Form\DataTransformer\ArrayToDelimitedStringTransformerTest::testSpaceAsDelimiter with data set #5 (array('foo', '', ''), 'foo  ', 'Empty elements at the end are preserved')
Empty elements at the end are preserved
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-foo  
+foo

symfony-form-array-to-delimited-string-transformer/tests/ArrayToDelimitedStringTransformerTest.php:132
                                       
FAILURES!                              
Tests: 19, Assertions: 23, Failures: 4.
```

Do we fix that?