Fix incorrect variable when setting `custom_metadata`

The variable `metadata_struct` isn’t defined in `#create_metadata`,
which (obviously) would explode, though it wasn’t immediately clear
from the thrown exception and stacktrace, as it was something along
the lines of:

	ERROR NoMethodError: undefined method `md5' for nil:NilClass

Which is due to FileStore returning `nil` from `#store_object` because
`metadata_struct` is `nil` which leads to yet another NoMethodError
being thrown, etc, etc. This should fix that though.
Specify sub second precision for ISO 8601 dates

This is needed for jets3t (v0.7.2) to work with fakes3, due to some
overly strict date parsing on their side. Hopefully this shouldn’t
affect interaction with any other clients, the tests are passing at
least.
Extract sub second precision value to a constant

With a comment why we need to specify any sub second precision at all.