Patch SD library to allow re-initialization after power down

[fakufaku]

This is a patch of the SD library to allow the SD card to be re-initialized after having been powered off.

This patch fixes issue 1103.

The problem was that the root directory is still marked as open and thus its initialization fails. The fix changes SD.init to first check if the root dir is open, and if so, closes it.

[facchinm]

@sandeepmistry since you recently updated SD library, do you think this might still be interesting?

[sandeepmistry]

@facchinm yes!

@fakufaku it would be great to have a sketch that can reproduce the issue.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}