Fix incorrect cache behavior in AssetServlet (#3736)

###### Problem:

AssetServlet is returning 304 when the `If-None-Match` header matches *OR* `If-Modified-Since` header is later or equal to the asset modified time.

This is incorrect according to [MDN's documentation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since)

###### Solution:

Based on the documentation if the request contains the `If-None-Match` header then they are compared instead of checking `If-Modified-Since`.

Fixes #3724

dropwizard-servlets/src/main/java/io/dropwizard/servlets/assets/AssetServlet.java

@@ -302,8 +302,14 @@ protected byte[] readResource(URL requestedResourceURL) throws IOException {
     }
 
     private boolean isCachedClientSide(HttpServletRequest req, CachedAsset cachedAsset) {
-        return cachedAsset.getETag().equals(req.getHeader(IF_NONE_MATCH)) ||
-                (req.getDateHeader(IF_MODIFIED_SINCE) >= cachedAsset.getLastModifiedTime());
+        // https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since 
+        // Indicates that with the presense of If-None-Match If-Modified-Since should be ignored.
+        String ifNoneMatchHeader = req.getHeader(IF_NONE_MATCH);
+        if (ifNoneMatchHeader != null) {
+            return cachedAsset.getETag().equals(ifNoneMatchHeader);
+        } else {
+            return req.getDateHeader(IF_MODIFIED_SINCE) >= cachedAsset.getLastModifiedTime();
+        }
     }
 
     /**

dropwizard-servlets/src/test/java/io/dropwizard/servlets/assets/AssetServletTest.java

@@ -150,6 +150,25 @@ void servesFilesFromRootsWithSameName() throws Exception {
                 .isEqualTo("HELLO THERE 2");
     }
 
+    @Test
+    void cacheIfModifiedSinceOverwrittenByIfNoneMatch() throws Exception{
+        request.setHeader(HttpHeader.IF_MODIFIED_SINCE.toString(), "Sat, 05 Nov 1955 22:57:05 GMT"); 
+        request.setURI(DUMMY_SERVLET + "index.htm");
+        response = HttpTester.parseResponse(SERVLET_TESTER.getResponses(request.generate()));
+
+        assertThat(response.getStatus())
+            .isEqualTo(200);
+
+        String eTag = response.get(HttpHeader.ETAG);
+
+        // If-None-Match should override If-Modified-Since
+        request.setHeader(HttpHeader.IF_NONE_MATCH.toString(), eTag);
+        response = HttpTester.parseResponse(SERVLET_TESTER.getResponses(request.generate()));
+        assertThat(response.getStatus())
+            .isEqualTo(304);
+
+    }
+
     @Test
     void servesFilesWithA200() throws Exception {
         response = HttpTester.parseResponse(SERVLET_TESTER.getResponses(request.generate()));