Improve request parsing with invalid json

[moritz-h]

Slim throws a runtime exception if json Request sends "semi valid" json code.

Here you expect json parsing to be an object, array or null:
https://github.com/slimphp/Slim/blob/3.x/Slim/Http/Request.php#L1015

If i send invalid json everything is fine, because the result ist null. If i send "semi valid" json like "foobar"
the runtime exception gets triggered.
The Problem is that the php function json_decode (called here) decodes a little more than valid json as mentioned here:
http://php.net/manual/en/function.json-decode.php
So "foobar" gets decoded as string, which is not an object, array or null.

I think ether you should accept "semi valid" json as result like php does or you should handle it like invalid json (i think better). But a RuntimeException looks totally wrong here.

I don't know how this should be fixed. The pull request just has tests for what i would expect.

[coveralls]

Coverage remained the same at 97.258% when pulling 368f978 on moritz-h:3.x into 66ff172 on slimphp:3.x.

[akrabat]

Thanks. I've also fixed the bug.