Circular reference on column object causes SystemStackError when serializing from hash

I'm not sure if this is exactly what is happening in https://github.com/lomba/schema_plus/issues/90, but I was having a problem with my application hanging after requiring the schema_plus gem.  

I tracked it down and figured out that I had a place (actually in the meta_request gem) that was calling ```to_json``` on a data structure that deep down in it contained a ```ActiveRecord::ConnectionAdapters::PostgreSQLColumn``` column object.  Looking at that object, I realized that there is a circular reference to the object that causes problems on serialization.

To illustrate the problem.

Without schema_plus, here is the output:

```ruby
puts User.columns.first.to_json
# {"name":"id","sql_type":"integer","null":false,"limit":null,"precision":null,"scale":null,"type":"integer","default":null,"primary":true,"coder":null}
```

With schema_plus, here is the output:

```ruby
puts User.columns.first.to_json
# ActiveSupport::JSON::Encoding::CircularReferenceError: object references itself
```

Now, if we put the column in to the hash (as it was in my case), we get

```ruby
{foo: User.columns.first}.to_json
# SystemStackError
```

This pull request contains a simple unit test that illustrates the problem.