v3(services): Ensure that whole credential binding view is filtered on

Without .from_self, there WHERE clause is only applied to the first
table in the view

[#173148626](https://www.pivotaltracker.com/story/show/173148626)

Author: Brian Butz

app/models/services/service_credential_binding_view.rb

@@ -25,7 +25,7 @@ module Types
         SERVICE_BINDING_VIEW
       ].inject do |statement, sub_select|
         statement.union(sub_select, all: true, from_self: false)
-      end.freeze
+      end.from_self.freeze
 
       class View < Sequel::Model(VIEW)
         many_to_one :service_instance, class: 'VCAP::CloudController::ServiceInstance'

spec/unit/fetchers/service_credential_binding_fetcher_spec.rb

@@ -7,6 +7,8 @@ module CloudController
       let(:fetcher) { ServiceCredentialBindingFetcher.new }
 
       describe 'not a real guid' do
+        let!(:existing_credential_binding) { ServiceBinding.make }
+
         it 'should return nothing' do
           credential_binding = fetcher.fetch('does-not-exist')
           expect(credential_binding).to be_nil