Fix/handle invalid wp json response

[alexsanford]

Fixes #1817

Testing instructions

Add the following code snippet to the WordPress site to simulate an unusual string response from the JSON API.

add_action( 'rest_api_init', function( $wp_rest_server ) { die( '404' ); } );

Run WPScan against the site and try to enumerate users (-e u). It should not crash.

[coveralls]

coverage: 85.984% (+0.02%) from 85.966%
when pulling d2841db on fix/handle-invalid-wp-json-response
into 96b6b81 on master.