Only use onHeadersReceived listener when necessary

As mentioned in #214

lib/dataUrls.js

@@ -41,11 +41,10 @@
 	}
 	
 	scope.init = function(){
-		browser.webRequest.onHeadersReceived.addListener(
+		function listener(details){console.log("listener");
-			function(details){
-				const cspMatch = (blockBlob? "": "blob: ") + "filesystem: *";
 			const headers = details.responseHeaders;
 			if (settings.blockDataURLs){
+				const cspMatch = (blockBlob? "": "blob: ") + "filesystem: *";
 				logging.verbose("Adding CSP header to", details);
 				setHeader(headers, {
 					name: "Content-Security-Policy",
@@ -56,13 +55,33 @@
 			return {
 				responseHeaders: headers
 			};
-			},
+		}
+		function addListener(){
+			if (!browser.webRequest.onHeadersReceived.hasListener(listener)){
+				browser.webRequest.onHeadersReceived.addListener(
+					listener,
 					{
 						urls: ["<all_urls>"],
 						types: ["main_frame", "sub_frame", "object"]
 					},
 					["blocking", "responseHeaders"]
 				);
+			}
+		}
+		function removeListener(){
+			browser.webRequest.onHeadersReceived.removeListener(listener);
+		}
+		function adjustListener(){
+			if (settings.blockDataURLs){
+				addListener();
+			}
+			else {
+				removeListener();
+			}
+		}
+		
+		settings.onloaded(adjustListener);
+		settings.on("blockDataURLs", adjustListener);
 	};
 
 }());

test/dataUrlTest.php

@@ -29,7 +29,7 @@
 	<iframe id="blobIframe"></iframe>
 	<h1>Data-URL object</h1>
 	<object
-		type="invalid"
+		type="text/html"
 		data="data:invalid;base64&#x2c;<?php
 			echo base64_encode(
 				str_replace(
@@ -42,7 +42,7 @@
 	></object>
 	<h1>Data-URL embed</h1>
 	<embed
-		type="invalid"
+		type="text/html"
 		src="data:invalid;base64&#x2c;<?php
 			echo base64_encode(
 				str_replace(