Add basic specs for `ProxyingConnectionPool`
user
```ruby
require 'spec_helper'

describe ActiveReplicas::ProxyingConnectionPool do
  subject { ActiveReplicas::ProxyingConnectionPool }

  before do
    @primary_pool = double 'primary connection pool'
    @replica_pool = double 'replica connection pool'

    allow(subject).to receive(:connection_pool_for_spec) do |spec|
      case spec
      when :primary then @primary_pool
      when :replica then @replica_pool
      else raise "Un-stubbed connection specification: #{spec}"
      end
    end

    @subject = subject.new primary: :primary,
                           replicas: { default0: :replica }
  end

  describe '#initialize' do
    it 'sets up its primary and replica pools' do
      expect(@subject.primary_pool).to eq @primary_pool
      expect(@subject.replica_pools[:default0]).to eq @replica_pool
    end
  end

  describe '#connection' do
    it 'returns a connection from the current pool' do
      connection = double 'connection'
      expect(@replica_pool).to receive(:connection).and_return(connection)
      expect(@subject.connection).to_not be_nil
    end

    it 'returns a proxied connection' do
      connection = double 'connection'
      @replica_pool.stub connection: connection

      proxying_connection = @subject.connection
      expect(proxying_connection).to be_a ActiveReplicas::ProxyingConnection
      expect(proxying_connection.proxied_connection).to eq connection
    end
  end

  describe '#current_pool' do
    it 'defaults to the next replica pool' do
      expect(@subject.current_pool).to eq @replica_pool
    end

    it 'respects overrides' do
      @primary_pool.stub connection: double('connection')

      @subject.with_primary do
        expect(@subject.current_pool).to eq @primary_pool
      end
    end
  end
end
```
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