Previously, MustacheViewResolver would create an InputStreamReader
that wraps the template Resource's InputStream but would fail to close
the Reader. When the InputStream was a FileInputStream, this caused
the resolver to leak file handles.

This commit updates the resolver to close the Reader once the Template
has been compiled, thereby allowing any underlying resources to be
cleaned up immediately, rather than having to wait for the JVM to exit.

Closes gh-4921