Adds null check in quoteIfNeeded

fixes gh-337

spring-cloud-gateway-core/src/main/java/org/springframework/cloud/gateway/filter/headers/ForwardedHeadersFilter.java

@@ -147,7 +147,7 @@ public class ForwardedHeadersFilter implements HttpHeadersFilter, Ordered {
 
 
 		private String quoteIfNeeded(String s) {
-			if (s.contains(":")) { //TODO: broaded quote
+			if (s != null && s.contains(":")) { //TODO: broaded quote
 				return "\""+s+"\"";
 			}
 			return s;

spring-cloud-gateway-core/src/test/java/org/springframework/cloud/gateway/filter/headers/ForwardedHeadersFilterTests.java

@@ -67,6 +67,29 @@ public class ForwardedHeadersFilterTests {
 				.containsEntry("for", "\"10.0.0.1:80\"");
 	}
 
+	@Test
+	public void noHostHeader() throws UnknownHostException {
+		MockServerHttpRequest request = MockServerHttpRequest
+				.get("http://localhost/get")
+				.remoteAddress(new InetSocketAddress(InetAddress.getByName("10.0.0.1"), 80))
+				.build();
+
+		ForwardedHeadersFilter filter = new ForwardedHeadersFilter();
+
+		HttpHeaders headers = filter.filter(request.getHeaders(), MockServerWebExchange.from(request));
+
+		assertThat(headers.get(FORWARDED_HEADER)).hasSize(1);
+
+		List<Forwarded> forwardeds = ForwardedHeadersFilter.parse(headers.get(FORWARDED_HEADER));
+
+		assertThat(forwardeds).hasSize(1);
+		Forwarded forwarded = forwardeds.get(0);
+
+		assertThat(forwarded.getValues())
+				.containsEntry("proto", "http")
+				.containsEntry("for", "\"10.0.0.1:80\"");
+	}
+
 	@Test
 	public void forwardedParsedCorrectly() {
 		String[] valid = new String[] {