[FEATURE] Validate that custom parameters have valid names

We could have an access checker, which does that kind of validation. Seems to be a bit of a better semantic place to do so.

+1

One step closer to full compliance with the standard! Thanks @dawehner. This is almost done. I added a couple of minor suggestions.

1. +++ b/src/Access/CustomParameterNames.php
   @@ -0,0 +1,40 @@
   +  public function access(Request $request) {
   

   Let's have a docblock with {@inheritdoc}'s.

2. +++ b/src/Access/CustomParameterNames.php
   @@ -0,0 +1,40 @@
   +  protected function validate(array $json_api_params) {
   

   Let's have a docblock with {@inheritdoc}'s.

3. +++ b/tests/src/Unit/Access/CustomParameterNamesTest.php
   @@ -0,0 +1,90 @@
   +class CustomParameterNamesTest extends \PHPUnit_Framework_TestCase {
   

   Is there any reason not to use Drupal's base UnitTestCase?

4. +++ b/tests/src/Unit/Access/CustomParameterNamesTest.php
   @@ -0,0 +1,90 @@
   +  public function testJsonApiParamsValidation($name, $valid) {
   

   Please add the @covers ::access annotation

5. +++ b/tests/src/Unit/Access/CustomParameterNamesTest.php
   @@ -0,0 +1,90 @@
   +      '/',
   ...
   +      // @todo vertical line?
   

   Maybe |?

6. +++ b/tests/src/Unit/Access/CustomParameterNamesTest.php
   @@ -0,0 +1,90 @@
   +      // @todo DELETE
   +      // @todo U+0000 to U+001F
   

   Not sure if you can check those into a text file in git. Let's leave them out.

This looks perfect to me. We just need a re-roll.

Re-roll.

Fixed and merged.