Bumping coverage on schema to 100%

I think it should be pretty much bullet proof at the moment (until the next failure). Also added external index lookups from main index for external docs. found this bug after refining lookups in schemas.

index/spec_index.go

@@ -495,6 +495,11 @@ func (index *SpecIndex) GetAllOperationsServers() map[string]map[string][]*Refer
 	return index.opServersRefs
 }
 
+// GetAllExternalIndexes will return all indexes for external documents
+func (index *SpecIndex) GetAllExternalIndexes() map[string]*SpecIndex {
+	return index.externalSpecIndex
+}
+
 func (index *SpecIndex) checkPolymorphicNode(name string) (bool, string) {
 	switch name {
 	case "anyOf":