do_task should return the task's return value

I would expect do_task() to return the task's return value, just like run_task() does.

$ rex -v
(R)?ex 1.2.1

$ uname -a
Darwin user.local 14.3.0 Darwin Kernel Version 14.3.0: Mon Mar 23 11:59:05 PDT 2015; root:xnu-2782.20.48~5/RELEASE_X86_64 x86_64

Consider the following test:

# file: return_values.t
use Rex -base;
use Test::More tests => 3;

my $got_return_value;
my $expected_return_value = { foo => "bar" };

task "foo" => sub {
  return { foo => "bar" };
};

$got_return_value = run_task "foo";
is_deeply( $got_return_value, $expected_return_value, "Calling task with run_task() returns the task's return value" );

$got_return_value = do_task "foo";
is_deeply( $got_return_value, $expected_return_value, "Calling task with do_task() returns the task's return value" );

$got_return_value = foo();
is_deeply( $got_return_value, $expected_return_value, "Calling task by function returns the task's return value" );

$ perl return_values.t
1..3
ok 1 - Calling task with run_task() returns the task's return value
[2015-05-26 11:56:20] INFO - Running task foo on <local>
not ok 2 - Calling task with do_task() returns the task's return value
#   Failed test 'Calling task with do_task() returns the task's return value'
#   at return_values.t line 15.
#     Structures begin differing at:
#          $got = undef
#     $expected = HASH(0x7fc088804638)
[2015-05-26 11:56:20] INFO - Running task foo on current connection
ok 3 - Calling task by function returns the task's return value
# Looks like you failed 1 test of 3.

Start with the return_values.t example and compare the documented behavior of run_task(), do_task(), and calling the task as a function. Run the three assertions shown in the issue; done means all three return the expected { foo => "bar" } value without changing the other task behavior.