fix coercion of ShallowAttributes values

[morr]

This PR fixes such cases:

class A
  include ShallowAttributes
  attribute :a, Integer
end

class B
  include ShallowAttributes
  attribute :b, Array, of: A
end
  
a = A.new(a: 1)
b = B.new(b: [a])

NoMethodError: undefined method `each_pair' for #<A a=1>
from /Users/morr/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/shallow_attributes-0.9.4/lib/shallow_attributes/instance_methods.rb:90:in `attributes='

[coveralls]

Coverage decreased (-0.1%) to 99.758% when pulling 6008302 on morr:fix-coercion into 37fe194 on davydovanton:master.

[coveralls]

Coverage increased (+0.002%) to 99.881% when pulling 9f0b031 on morr:fix-coercion into 37fe194 on davydovanton:master.

[davydovanton]

Code looks good. Could you add some tests for this bug?

[morr]

@davydovanton done

[lmmendes]

It seems that pull request #32 also addresses this issue.