Honor NoTrimSpace field for flags from env

This commits addresses the following comment:
#1649 (comment)

String slice flag values from environment now follows the same rules as
parsed values.

flag_string_slice.go

@@ -157,7 +157,10 @@ func (f *StringSliceFlag) Apply(set *flag.FlagSet) error {
 
 	if val, source, found := flagFromEnvOrFile(f.EnvVars, f.FilePath); found {
 		for _, s := range f.separator.flagSplitMultiValues(val) {
-			if err := setValue.Set(strings.TrimSpace(s)); err != nil {
+			if !f.NoTrimSpace {
+				s = strings.TrimSpace(s)
+			}
+			if err := setValue.Set(s); err != nil {
 				return fmt.Errorf("could not parse %q as string value from %s for flag %s: %s", val, source, f.Name, err)
 			}
 		}

flag_test.go

@@ -144,6 +144,11 @@ func TestFlagsFromEnv(t *testing.T) {
 		s.hasBeenSet = false
 		return *s
 	}
+	newSetStringSliceNoTrimSpace := func(defaults ...string) StringSlice {
+		s := newSetStringSlice(defaults...)
+		s.noTrimSpace = true
+		return s
+	}
 
 	var flagTests = []struct {
 		input     string
@@ -198,6 +203,8 @@ func TestFlagsFromEnv(t *testing.T) {
 		{"path", "path", &PathFlag{Name: "path", EnvVars: []string{"PATH"}}, ""},
 
 		{"foo,bar", newSetStringSlice("foo", "bar"), &StringSliceFlag{Name: "names", EnvVars: []string{"NAMES"}}, ""},
+		{" space ", newSetStringSliceNoTrimSpace(" space "), &StringSliceFlag{Name: "names", NoTrimSpace: true, EnvVars: []string{"NAMES"}}, ""},
+		{" no space ", newSetStringSlice("no space"), &StringSliceFlag{Name: "names", EnvVars: []string{"NAMES"}}, ""},
 
 		{"1", uint(1), &UintFlag{Name: "seconds", EnvVars: []string{"SECONDS"}}, ""},
 		{"08", uint(8), &UintFlag{Name: "seconds", EnvVars: []string{"SECONDS"}, Base: 10}, ""},