Add some unit tests for lib/api/helpers.rb
user
```ruby
require 'spec_helper'

describe API::Helpers do
  subject { Class.new.include(described_class).new }

  describe '#find_namespace' do
    let(:namespace) { create(:namespace) }

    shared_examples 'namespace finder' do
      context 'when namespace exists' do
        it 'returns requested namespace' do
          expect(subject.find_namespace(existing_id)).to eq(namespace)
        end
      end

      context "when namespace doesn't exists" do
        it 'returns nil' do
          expect(subject.find_namespace(non_existing_id)).to be_nil
        end
      end
    end

    context 'when ID is used as an argument' do
      let(:existing_id) { namespace.id }
      let(:non_existing_id) { 9999 }

      it_behaves_like 'namespace finder'
    end

    context 'when PATH is used as an argument' do
      let(:existing_id) { namespace.path }
      let(:non_existing_id) { 'non-existing-path' }

      it_behaves_like 'namespace finder'
    end
  end

  shared_examples 'user namespace finder' do
    let(:user1) { create(:user) }

    before do
      allow(subject).to receive(:current_user).and_return(user1)
      allow(subject).to receive(:header).and_return(nil)
      allow(subject).to receive(:not_found!).and_raise('404 Namespace not found')
    end

    context 'when namespace is group' do
      let(:namespace) { create(:group) }

      context 'when user has access to group' do
        before do
          namespace.add_guest(user1)
          namespace.save!
        end

        it 'returns requested namespace' do
          expect(namespace_finder).to eq(namespace)
        end
      end

      context "when user doesn't have access to group" do
        it 'raises not found error' do
          expect { namespace_finder }.to raise_error(RuntimeError, '404 Namespace not found')
        end
      end
    end

    context "when namespace is user's personal namespace" do
      let(:namespace) { create(:namespace) }

      context 'when user owns the namespace' do
        before do
          namespace.owner = user1
          namespace.save!
        end

        it 'returns requested namespace' do
          expect(namespace_finder).to eq(namespace)
        end
      end

      context "when user doesn't own the namespace" do
        it 'raises not found error' do
          expect { namespace_finder }.to raise_error(RuntimeError, '404 Namespace not found')
        end
      end
    end
  end

  describe '#find_namespace!' do
    let(:namespace_finder) do
      subject.find_namespace!(namespace.id)
    end

    it_behaves_like 'user namespace finder'
  end

  describe '#user_namespace' do
    let(:namespace_finder) do
      subject.user_namespace
    end

    before do
      allow(subject).to receive(:params).and_return({ id: namespace.id })
    end

    it_behaves_like 'user namespace finder'
  end
end
```
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