ruby: add bindings for `notmuch_database_get_all_tags`

The Ruby bindings were missing a way to get all the tags of the
database. Now you should be able to access this with the public
instance method `all_tags` of your database object.

Example of use:
    notmuchdb = Notmuch::Database.new path, { :create => false,
    	:mode => Notmuch::MODE_READ_ONLY }

    my_tags = notmuchdb.all_tags

    my_tags.each { |tag|
      print tag
    }

    my_tags.destroy!

Amended by db: improve error reporting, add test

1 files changed, 16 insertions, 0 deletions

diff --git a/test/T395-ruby.sh b/test/T395-ruby.sh
index d5cade8f..20e06917 100755
--- a/test/T395-ruby.sh
+++ b/test/T395-ruby.sh
@@ -83,4 +83,20 @@ EOF
 notmuch count --output=threads tag:inbox > EXPECTED
 test_expect_equal_file OUTPUT EXPECTED
 
+test_begin_subtest "get all tags"
+test_ruby <<"EOF"
+require 'notmuch'
+$maildir = ENV['MAIL_DIR']
+if not $maildir then
+  abort('environment variable MAIL_DIR must be set')
+end
+@db = Notmuch::Database.new($maildir)
+@t = @db.all_tags()
+for tag in @t do
+   print tag,"\n"
+end
+EOF
+notmuch search --output=tags '*' > EXPECTED
+test_expect_equal_file OUTPUT EXPECTED
+
 test_done