Return immediately if request is not json

[fracz]

Fetching the contents before this checkick if the request is json may result in OutOfMemory excpetion if the request is multipart and contains big files.

Sample exception excerpt:

OutOfMemoryException in Request.php line 1564:
Error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 67108888 bytes)
in Request.php line 1564
at file_get_contents() in Request.php line 1564
at Request->getContent() in JsonRequestTransformerListener.php line 30
at JsonRequestTransformerListener->onKernelRequest() in WrappedListener.php line 106

Checking if request contains json content type headers before fetching the contents prevents this error.

[fracz]

These fails are not introduced by this PR... Something in PHPUnit has changed. https://travis-ci.org/qandidate-labs/symfony-json-request-transformer/jobs/225105494

[othillo]

@fracz the build fails because Travis CI uses PHPUnit 6. This PR should fix it: #14

[othillo]

thank you, @fracz !