add new EdgeThrottled exception

Edge throttles *much* more aggressively than other browsers, added
a new ResponseError subclass called EdgeThrottled and added code
to verify_response to distinguish this case from a generic
ResponseError. I'd still like to know if I ever get throttled by
Chrome, Firefox, or Safari since it's never happened before, but
it happens so much with Edge that I've just given up and ignore
these exceptions personally. Others may want to handle this
specific case differently as well.

lib/web_push/errors.rb

@@ -26,4 +26,6 @@ class PayloadTooLarge < ResponseError; end
   class TooManyRequests < ResponseError; end
 
   class PushServiceError < ResponseError; end
+
+  class EdgeThrottled < ResponseError; end
 end

lib/web_push/request.rb

@@ -147,6 +147,8 @@ def verify_response(resp)
       elsif resp.is_a?(Net::HTTPUnauthorized) || resp.is_a?(Net::HTTPForbidden) || # 401, 403
             resp.is_a?(Net::HTTPBadRequest) && resp.message == 'UnauthorizedRegistration' # 400, Google FCM
         raise Unauthorized.new(resp, uri.host)
+      elsif resp.is_a?(Net::HTTPNotAcceptable) && uri.host.to_s.ends_with?("notify.windows.com") #406 on Edge
+        raise EdgeThrottled.new(resp, uri.host)
       elsif resp.is_a?(Net::HTTPRequestEntityTooLarge) # 413
         raise PayloadTooLarge.new(resp, uri.host)
       elsif resp.is_a?(Net::HTTPTooManyRequests) # 429, try again later!