Escape schema names in PostgresqlSchemaAdapter

Currently, we can create a new schema with a name containing dashes or dots, but we can't switch to this schema. For example
```ruby
> Apartment::Database.create('test-database')
  (18.0ms)  CREATE SCHEMA "test-database"
  Apartment::SchemaNotFound: One of the following schema(s) is invalid: test-database
  from .../lib/apartment/adapters/postgresql_adapter.rb:82:in `rescue in connect_to_new'
```

This PR adds double quotes around schema names in full_schema_path method in PostgresqlSchemaAdapter (and in corresponding tests) to correct this bug.