Fix SQL explains in developer mode

After the database metrics were added to the Newrelic web service,
we've been seeing errors when trying to view explains and backtraces
in developer mode. This change adjusts the array manipulation that
developer mode's explain_sql action uses to match the new format
returned by @segment.explain_sql.

lib/new_relic/rack/developer_mode.rb

@@ -83,9 +83,9 @@ def explain_sql
           @obfuscated_sql = @segment.obfuscated_sql
         end
 
-        explanations = @segment.explain_sql
+        headers, explanations = @segment.explain_sql
         if explanations
-          @explanation = explanations.first
+          @explanation = explanations
           if !@explanation.blank?
             first_row = @explanation.first
             # Show the standard headers if it looks like a mysql explain plan

test/new_relic/rack/developer_mode_test.rb

@@ -40,4 +40,35 @@ def test_show_sample_summary_displays_sample_details
     assert last_response.body.include?('SandwichesController')
     assert last_response.body.include?('index')    
   end  
+
+  def test_explain_sql_displays_query_plan
+    sample = @sampler.samples[0]
+    sql_segment = sample.sql_segments[0]
+    explain_results = NewRelic::Agent::Database.process_resultset(example_explain_as_hashes)
+
+    NewRelic::TransactionSample::Segment.any_instance.expects(:explain_sql).returns(explain_results)
+    get "/newrelic/explain_sql?id=#{sample.sample_id}&segment=#{sql_segment.segment_id}"
+
+    assert last_response.ok?
+    assert last_response.body.include?('PRIMARY')
+    assert last_response.body.include?('Key Length')
+    assert last_response.body.include?('Using index')
+  end
+
+  private
+
+  def example_explain_as_hashes
+    [{
+      'Id' => '1',
+      'Select Type' => 'SIMPLE',
+      'Table' => 'sandwiches',
+      'Type' => 'range',
+      'Possible Keys' => 'PRIMARY',
+      'Key' => 'PRIMARY',
+      'Key Length' => '4',
+      'Ref' => '',
+      'Rows' => '1',
+      'Extra' => 'Using index'
+    }]
+  end
 end