accounts/abi: handle the "constant" modifier for functions

accounts/abi/abi.go

@@ -270,7 +270,7 @@ func (abi *ABI) UnmarshalJSON(data []byte) error {
 	var fields []struct {
 		Type     string
 		Name     string
-		Const   bool
+		Constant bool
 		Indexed  bool
 		Inputs   []Argument
 		Outputs  []Argument
@@ -288,7 +288,7 @@ func (abi *ABI) UnmarshalJSON(data []byte) error {
 		case "function", "":
 			abi.Methods[field.Name] = Method{
 				Name:    field.Name,
-				Const:   field.Const,
+				Const:   field.Constant,
 				Inputs:  field.Inputs,
 				Outputs: field.Outputs,
 			}

accounts/abi/method.go

@@ -67,8 +67,11 @@ func (m Method) String() string {
 		}
 		outputs[i] += output.Type.String()
 	}
-
-	return fmt.Sprintf("function %v(%v) returns(%v)", m.Name, strings.Join(inputs, ", "), strings.Join(outputs, ", "))
+	constant := ""
+	if m.Const {
+		constant = "constant "
+	}
+	return fmt.Sprintf("function %v(%v) %sreturns(%v)", m.Name, strings.Join(inputs, ", "), constant, strings.Join(outputs, ", "))
 }
 
 func (m Method) Id() []byte {