Fix HEAD requests for kernelspec resources

Closes gh-7237
Closes gh-7258

StaticFileHandler.get() is a coroutine. When Tornado calls a handler
method, it uses the return value to determine whether or not it's a
coroutine. So when head() calls get(), it needs to pass the return value
on for Tornado to handle it properly.
Merge pull request #7259 from takluyver/kernelspec-head-error

Fix HEAD requests for kernelspec resources