add integration with SimpleForm's `input_field`

closes #102
do not try to set default value for not selected attributes
Merge pull request #103 from brainspec/issue-101

do not try to set default value for not selected attributes
Update CHANGELOG.md
Fix RSpec matcher description
Merge pull request #104 from andreygerasimchuk/rspec-description-fix

Fix RSpec matcher description
add spec for rspec matcher description
update CHANGELOG [ci skip]
update CHANGELOG [ci skip]
Update README.md