Workaround for hub4j#669 - remove If-Modified-Since header

This is a first cut at working round hub4j#669.  It is hacky as hell and I hate it.

src/main/java/org/kohsuke/github/extras/okhttp3/ObsoleteUrlFactory.java

@@ -19,6 +19,7 @@
 import okio.Okio;
 import okio.Pipe;
 import okio.Timeout;
+import org.jetbrains.annotations.NotNull;
 
 /*
  * Copyright (C) 2014 Square, Inc.
@@ -617,7 +618,18 @@ private Call buildCall() throws IOException {
             OkHttpClient.Builder clientBuilder = client.newBuilder();
             clientBuilder.interceptors().clear();
             clientBuilder.interceptors().add(UnexpectedException.INTERCEPTOR);
+
             clientBuilder.networkInterceptors().clear();
+
+            // network interceptors are generally not allowed
+            // (probably buggy when used with this hacked in adapter)
+            // but we have to use this one to handle github issue:
+            // https://github.com/isaacs/github/issues/692
+            // #669 in this project
+            if (client.cache() != null && getUseCaches()) {
+                clientBuilder.addNetworkInterceptor(new OkHttpConnector.RemoveIfModifiedSinceRequestHeader());
+            }
+
             clientBuilder.networkInterceptors().add(networkInterceptor);
 
             // Use a separate dispatcher so that limits aren't impacted. But use the same executor service!

src/main/java/org/kohsuke/github/extras/okhttp3/OkHttpConnector.java

@@ -1,5 +1,8 @@
 package org.kohsuke.github.extras.okhttp3;
 
+import okhttp3.Interceptor;
+import okhttp3.Request;
+import okhttp3.Response;
 import okhttp3.CacheControl;
 import okhttp3.ConnectionSpec;
 import okhttp3.OkHttpClient;
@@ -51,12 +54,18 @@ public OkHttpConnector(OkHttpClient client, int cacheMaxAge) {
         OkHttpClient.Builder builder = client.newBuilder();
 
         builder.connectionSpecs(TlsConnectionSpecs());
-        this.client = builder.build();
-        if (cacheMaxAge >= 0 && this.client != null && this.client.cache() != null) {
+
+        if (cacheMaxAge >= 0 && client.cache() != null) {
             maxAgeHeaderValue = new CacheControl.Builder().maxAge(cacheMaxAge, TimeUnit.SECONDS).build().toString();
+            // HttpURLConnection does not support networkInterceptors, so this would not work
+            // However, we hacked ObsoleteUrlFactory to do this automatically for us.
+            // builder.addNetworkInterceptor(new RemoveIfModifiedSinceRequestHeader());
         } else {
             maxAgeHeaderValue = null;
         }
+
+        this.client = builder.build();
+
         this.urlFactory = new ObsoleteUrlFactory(this.client);
     }
 
@@ -79,4 +88,17 @@ public HttpURLConnection connect(URL url) throws IOException {
     private List<ConnectionSpec> TlsConnectionSpecs() {
         return Arrays.asList(ConnectionSpec.MODERN_TLS, ConnectionSpec.CLEARTEXT);
     }
+
+    static class RemoveIfModifiedSinceRequestHeader implements Interceptor {
+        @Override
+        public Response intercept(Chain chain) throws IOException {
+            Request currentRequest = chain.request();
+            if (currentRequest.header("If-Modified-Since") != null) {
+                currentRequest = currentRequest.newBuilder()
+                    .removeHeader("If-Modified-Since")
+                    .build();
+            }
+            return chain.proceed(currentRequest);
+        }
+    }
 }