Missing field matching

#### Fix for missing fields in API call.

###### Changes.
* Added check to see see if the field Datamapper is trying to filter exists and passes it through if the field doesn't exist.

As you can see from the number of commits, I was trying a lot of possibilities. :blush: 
###### Full code change
```ruby
def matches?(record)
  if relationship? && expected.respond_to?(:query)
    match_relationship?(record)
  else
    if !record.has_key?(@subject.field) # This passes through the record with a missing filter field
      true
    else
      super
    end
  end
end
```