Add default http client initializing.

[MikDal002]

Hello!
I've tried to use this library and unfortunately came across the bug. Here is the fix :).

[daveaglick]

Thanks! So to make sure I understand the issue here, it's when you pass null into the constructor for an HttpClient? Seems like a nice defensive fix to check for that.

[MikDal002]

I used var myClient = new NetlifyClient("my-access-code");. It was enough to came across this problem :). I'm glad that you merged those changes :).