Fix PATCH method

The PATCH method for the entry should only update what user sent to us and not the whole entry as it was before.
Also, sending tags when patching an entry will now remove all current tags & assocatied new ones.

tests/Wallabag/CoreBundle/Helper/ContentProxyTest.php

@@ -221,12 +221,9 @@ class ContentProxyTest extends \PHPUnit_Framework_TestCase
             ->method('tag');
 
         $validator = $this->getValidator();
-        $validator->expects($this->exactly(2))
+        $validator->expects($this->once())
             ->method('validate')
-            ->will($this->onConsecutiveCalls(
-                new ConstraintViolationList([new ConstraintViolation('oops', 'oops', [], 'oops', 'language', 'dontexist')]),
-                new ConstraintViolationList()
-            ));
+            ->willReturn(new ConstraintViolationList([new ConstraintViolation('oops', 'oops', [], 'oops', 'language', 'dontexist')]));
 
         $graby = $this->getMockBuilder('Graby\Graby')
             ->setMethods(['fetchContent'])