Fix unresolved function call on JVM < 1.7

This notably allows the mockwebserver project to be run on Android.
ServerSocket only implements Closeable since Java 1.7 and thus calling of
closeQuietly(Closeable) with a ServerSocket argument fails when running
on Java 1.5 or 1.6 JVMs. This is the same as is already done for
closeQuietly(Socket).

okhttp-protocols/src/main/java/com/squareup/okhttp/internal/Util.java

@@ -26,6 +26,7 @@ import java.io.Reader;
 import java.io.StringWriter;
 import java.io.UnsupportedEncodingException;
 import java.net.Socket;
+import java.net.ServerSocket;
 import java.net.URI;
 import java.net.URL;
 import java.nio.ByteOrder;
@@ -136,6 +137,21 @@ public final class Util {
     }
   }
 
+  /**
+   * Closes {@code serverSocket}, ignoring any checked exceptions. Does nothing if
+   * {@code serverSocket} is null.
+   */
+  public static void closeQuietly(ServerSocket serverSocket) {
+    if (serverSocket != null) {
+      try {
+        serverSocket.close();
+      } catch (RuntimeException rethrown) {
+        throw rethrown;
+      } catch (Exception ignored) {
+      }
+    }
+  }
+
   /**
    * Closes {@code a} and {@code b}. If either close fails, this completes
    * the other close and rethrows the first encountered exception.