Fixed #10

[theroch]

Check if $http_response_header is set before using it.
This fixes #10

[waleedfarooq123]

Check if $http_response_header is set before using it. This fixes #10

Hi, will you please merge this branch into master

[tenzap]

The null coalescing operator (??) is new in PHP 7.0.
If one wants to support versions prior to it, like 5.6, maybe one should use a more compatible syntax.

Having issue #10 fixed would be nice though.

[theroch]

The minimum requirement for this lib is PHP 7.0.
Consideration of 5.6 and below is not necessary.