DataMapper::Collection +/- methods don't work with 'limit'

[devver]

For most options, +/i seems to work fine:

Project.all(:owner => 'bob') + Project.all(:private => true) # works as expected
Project.all(:owner => 'bob') - Project.all(:private => true) # works as expected

However, neither work if either or both have a 'limit' parameter:

Project.all(:limit => 5) - Project.all(:private => true)
# Hirb Error: InvalidQueryExpression: The specified query expression syntax is not valid.