Address :returning_id column should be of type Column

This pull request addresses isssue #134 and add a testcase for #252.

Eight of tests in `./spec/active_record/connection_adapters/oracle_enhanced_schema_statements_spec.rb` generate the error message below.

```ruby
Could not log "sql.active_record" event. NoMethodError: undefined method `name' for :returning_id:Symbol`
```
And #252 addresses this error message.

```ruby
 180       it "should return new key value using connection insert method" do
 181         insert_id = @conn.insert("INSERT INTO test_employees (first_name) VALUES ('Raimonds')", nil, "id")
 182         @conn.select_value("SELECT test_employees_seq.currval FROM dual").should == insert_id
 183       end
```

```ruby
 185       it "should create new record for model" do
 186         e = TestEmployee.create!(:first_name => 'Raimonds')
 187         @conn.select_value("SELECT test_employees_seq.currval FROM dual").should == e.id
 188       end
```

```ruby
 210       it "should return new key value using connection insert method" do
 211         insert_id = @conn.insert("INSERT INTO test_employees (first_name) VALUES ('Raimonds')", nil, "id")
 212         @conn.select_value("SELECT test_employees_seq.currval FROM dual").should == insert_id
 213       end
```

```ruby
 215       it "should create new record for model" do
 216         e = TestEmployee.create!(:first_name => 'Raimonds')
 217         @conn.select_value("SELECT test_employees_seq.currval FROM dual").should == e.id
 218       end
```

```ruby
 247       it "should return new key value using connection insert method" do
 248         insert_id = @conn.insert("INSERT INTO test_employees (first_name) VALUES ('Raimonds')", nil, @primary_key)
 249         @conn.select_value("SELECT #{@sequence_name}.currval FROM dual").should == insert_id
 250       end
```

```ruby
 252       it "should create new record for model with autogenerated sequence option" do
 253         e = TestEmployee.create!(:first_name => 'Raimonds')
 254         @conn.select_value("SELECT #{@sequence_name}.currval FROM dual").should == e.id
 255       end

```ruby
 283       it "should return new key value using connection insert method" do
 284         insert_id = @conn.insert("INSERT INTO test_employees (first_name) VALUES ('Raimonds')", nil, "id")
 285         @conn.select_value("SELECT #{@sequence_name}.currval FROM dual").should == insert_id
 286       end
```

```ruby
 288       it "should create new record for model with autogenerated sequence option" do
 289         e = TestEmployee.create!(:first_name => 'Raimonds')
 290         @conn.select_value("SELECT #{@sequence_name}.currval FROM dual").should == e.id
 291       end
```