Respect --document for `gem update`

The update command still uses Gem::DependencyInstaller for installing
gems (instead of Gem::RequestSet).  The DependencyInstaller adapter to
RequestSet did not pass along the document options so update fell back
to the default options.

This caused --no-document to be ignored.  Now these options are passed
down to RequestSet.

Fixes #946

Pair: @Elffers