Bugfix/15134 set response body (#15287)

* #15134

See #15075 and department-of-veterans-affairs/va.gov-team#65182 for the
Original PR and Github Issue, respectively.

This PR exposes a `body` attribute on the `GiBillStatusResponse` object, so it can be logged out as intended in the original PR.

lib/evss/gi_bill_status/gi_bill_status_response.rb

@@ -126,6 +126,10 @@ def error_type
         'unknown'
       end
 
+      def body
+        @response.body
+      end
+
       private
 
       def timeout?

spec/controllers/v0/post911_gi_bill_statuses_controller_spec.rb

@@ -136,11 +136,16 @@
       end
 
       it 'raises an InternalServerError including an exception with a detailed message' do
-        expect_block = expect do
+        response = instance_double(EVSS::GiBillStatus::GiBillStatusResponse)
+        allow(response).to receive(:error_type).and_return('unknown')
+        allow(response).to receive(:status).and_return(500)
+        allow(response).to receive(:body).and_return('Unknown error')
+
+        expectation = expect do
           controller.send(:render_error_json, response)
         end
 
-        expect_block.to raise_error(Common::Exceptions::InternalServerError) do |error|
+        expectation.to raise_error(Common::Exceptions::InternalServerError) do |error|
           expected_msg = 'An unknown error occurred. Response error type: unknown, status: 500, body: Unknown error'
           expect(error.exception.message).to eq(expected_msg)
         end