Register TypeMapperInterface and TypeMapper as one singleton

One can easily make a mistake by resolving `TypeMapper` from the container instead of `TypeMapperInterface`. This class wasn't registered as singleton (only the interface) so you always get a new instance without types. By registering them both as one singleton, we make it less error prone.

Closes: swisnl#65

Author: JaZo

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 * Reworded `ValidationException` messages to align them with the wordings used in the specification.
 * `DocumentParser` throws a `ValidationException` when it encounters duplicate resources.
+* `TypeMapperInterface` and `TypeMapper` are now registered as the same singleton.
 
 ## [1.0.0-beta.3] - 2019-09-30
 

src/Providers/ServiceProvider.php

@@ -37,7 +37,8 @@ public function boot()
 
     protected function registerSharedTypeMapper()
     {
-        $this->app->singleton(TypeMapperInterface::class, TypeMapper::class);
+        $this->app->bind(TypeMapperInterface::class, TypeMapper::class);
+        $this->app->singleton(TypeMapper::class);
     }
 
     protected function registerParsers()