Avoid falsy comparison

[nbayramberdiyev]

This PR fixes the comparison of the return value of strpos function.

If the needle string we are searching for is at the beginning of the haystack string, it will return 0 which is a valid offset and is falsy.

If you want to see a failing test, change this line like so:

- $response = $response->withStatus(404, "Not Found\n");
+ $response = $response->withStatus(404, "\nNot Found");

[coveralls]

Coverage remained the same at 99.621% when pulling e65ba7a on nbayramberdiyev:fix/strict-equivalence-operator into 4647c2a on slimphp:master.