v2 - Fix incorrectTypeForFlagError for unknowns

The `reflect` package doesn't always know what to call a type in
`(reflect.Type).Name()`, but `fmt.Errorf("%T")` always gets it right.
Use that so we get actionable error messages.

altsrc/map_input_source.go

@@ -3,7 +3,6 @@ package altsrc
 import (
 	"fmt"
 	"math"
-	"reflect"
 	"strings"
 	"time"
 
@@ -471,11 +470,5 @@ func (fsm *MapInputSource) isSet(name string) bool {
 }
 
 func incorrectTypeForFlagError(name, expectedTypeName string, value interface{}) error {
-	valueType := reflect.TypeOf(value)
-	valueTypeName := ""
-	if valueType != nil {
-		valueTypeName = valueType.Name()
-	}
-
-	return fmt.Errorf("Mismatched type for flag '%s'. Expected '%s' but actual is '%s'", name, expectedTypeName, valueTypeName)
+	return fmt.Errorf("Mismatched type for flag '%s'. Expected '%s' but actual is '%T'", name, expectedTypeName, value)
 }

altsrc/map_input_source_test.go

@@ -1,6 +1,7 @@
 package altsrc
 
 import (
+	"fmt"
 	"testing"
 	"time"
 )
@@ -33,3 +34,8 @@ func TestMapInputSource_Int64Slice(t *testing.T) {
 	expect(t, []int64{1, 2, 3}, d)
 	expect(t, nil, err)
 }
+
+func TestMapInputSource_IncorrectFlagTypeError(t *testing.T) {
+	var testVal *bool
+	expect(t, incorrectTypeForFlagError("test", "bool", testVal), fmt.Errorf("Mismatched type for flag 'test'. Expected 'bool' but actual is '*bool'"))
+}