[PHP] Correctly format JSON in headers

`ObjectSerializer::toHeaderValue()` in the generated PHP code calls
`toString()` on the values, which formats JSON with the `JSON_PRETTY_PRINT`
option. This will result in a multi-line header which cannot be parsed
since linebreaks aren't allowed by RFC 7230.

In my case I have a header schema called `UpdateUser` which I had hoped
would be serialized as `{"type":"staff","id":123}`.

Every single `__toString()` in the generator does the same thing, so I
figured it's safe to change `toHeaderValue()` to convert to JSON directly,
without `JSON_PRETTY_PRINT`. This fix works for me.

modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache

@@ -140,7 +140,7 @@ class ObjectSerializer
      */
     public static function toHeaderValue($value)
     {
-        return self::toString($value);
+       return json_encode(ObjectSerializer::sanitizeForSerialization($value));
     }
 
     /**

samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php

@@ -150,7 +150,7 @@ public static function toQueryValue($object)
      */
     public static function toHeaderValue($value)
     {
-        return self::toString($value);
+        return json_encode(ObjectSerializer::sanitizeForSerialization($value));
     }
 
     /**

samples/openapi3/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php

@@ -150,7 +150,7 @@ public static function toQueryValue($object)
      */
     public static function toHeaderValue($value)
     {
-        return self::toString($value);
+        return json_encode(ObjectSerializer::sanitizeForSerialization($value));
     }
 
     /**