Set writer modification time to now by default

CRuby does this lazily on first write, when it sets up the
headers, but that laziness is handled inside jzlib for us. Just
set it and if it is updated by user it will be updated in the
writer.

Fixes jruby#7879, which led to tests being skipped in
rack/rack#2182.

core/src/main/java/org/jruby/ext/zlib/JZlibRubyGzipWriter.java

@@ -134,8 +134,9 @@ private int processLevel(int argc, IRubyObject[] args, Ruby runtime) {
         return level;
     }
 
-    private IRubyObject initializeCommon(IRubyObject stream, int level) {
-        realIo = (RubyObject) stream;
+    private IRubyObject initializeCommon(ThreadContext context, IRubyObject stream, int level) {
+        Ruby runtime = context.runtime;
+        realIo = stream;
         try {
             // the 15+16 here is copied from a Deflater default constructor
             Deflater deflater = new Deflater(level, 15+16, false);
@@ -158,9 +159,15 @@ public void write(byte[] bytes, int off, int len) throws IOException {
             };
 
             io = new GZIPOutputStream(ioOutputStream, deflater, 512, false);
+
+            // set mtime to current time in case it is never updated
+            long now = System.currentTimeMillis();
+            this.mtime = RubyTime.newTime(runtime, now);
+            io.setModifiedTime(now / 1000);
+
             return this;
         } catch (IOException ioe) {
-            throw getRuntime().newIOErrorFromException(ioe);
+            throw runtime.newIOErrorFromException(ioe);
         }
     }