Fix getHeader to be case-insensitive

On certain BigCommerce stores, getHeader fails because the X-BC-ApiLimit-Remaining header is all lower-case in the HTTP response.  This causes getRequestsRemaining to always return 0 and puts the code into an infinite wait for more requests.  Instead, change getHeader to do a case-insensitive search for the header if the exact case is not found.  Please merge this into the trunk.

Author: atensoftware

src/Bigcommerce/Api/Connection.php

@@ -556,6 +556,12 @@ public function getHeader($header)
         if (array_key_exists($header, $this->responseHeaders)) {
             return $this->responseHeaders[$header];
         }
+        // Do case-insensitive search
+        foreach($this->responseHeaders as $k => $v) {
+            if(strtolower($k) == strtolower($header)) {
+                return $v;
+            }
+        }
     }
 
     /**