avoid blank hash in embeded parameter

[y00rb]

Hi Guys

1. improve the generate embedded attributes(has_many relation).if the has_many value is blank, will make errors in embeded_params, such as TypeError: no implicit conversion of nil into Hash, NoMethodError: undefined method[]=' for nil:NilClass`, so I have add method to make default in inject, set blank hash.
2. added has_one in to_params

[y00rb]

@remiprev Could you checkout it? if it is merge, I can used new version in my project