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).
Merge pull request #322 from desseim/master

Fix unresolved function call on JVM < 1.7