[SalesForce] Header names appear to be case-sensitive in HttpResponse

I've got NullPointerException trying to analyze the value of content type header returned in HttpResponse object.
As it turns out, the case of the header name was different (i.e. 'content-type', but not 'Content-Type'), that's why HttpResponse object returns null instead of actual value.
I'm really confused cause according to http specification header names are case-insensitive.

So I've created a test mock and below is a piece of code to reproduce the issue:

HttpRequest request = new HttpRequest();

request.setEndpoint('http://demo8399479.mockable.io/testHeaderCase');
request.setMethod('GET');

HttpResponse response = new Http().send(request);

System.debug('Get header content-type: ' + response.getHeader('content-type'));
System.debug('Get header Content-Type: ' + response.getHeader('Content-Type'));

And here is debug output:

20:59:12:131 USER_DEBUG [9]|DEBUG|Get header Content-Type: application/json; charset=UTF-8
20:59:12:131 USER_DEBUG [8]|DEBUG|Get header content-type: null

So, what's than the right way to fetch header by name ? Am I doing something wrong ? Why is it case-sensitive in apex ?

Thanks in advance for throwing daylight on this mystery.