file: fix whitespaces are filtered in the SaveTo method (#260)

Author: ygj6

file.go

@@ -454,6 +454,8 @@ func (f *File) writeToBuffer(indent string) (*bytes.Buffer, error) {
 					val = `"""` + val + `"""`
 				} else if !f.options.IgnoreInlineComment && strings.ContainsAny(val, "#;") {
 					val = "`" + val + "`"
+				} else if len(strings.TrimSpace(val)) != len(val) {
+					val = `"` + val + `"`
 				}
 				if _, err := buf.WriteString(equalSign + val + LineBreak); err != nil {
 					return nil, err

file_test.go

@@ -437,6 +437,25 @@ test   =
 `)
 
 	})
+
+	Convey("Keep leading and trailing spaces in value", t, func() {
+		f, _ := ini.Load([]byte(`[foo]
+bar1 = '  val ue1 '
+bar2 = """  val ue2 """
+bar3 = "  val ue3 "
+`))
+		So(f, ShouldNotBeNil)
+
+		var buf bytes.Buffer
+		_, err := f.WriteTo(&buf)
+		So(err, ShouldBeNil)
+		So(buf.String(),ShouldEqual,`[foo]
+bar1 = "  val ue1 "
+bar2 = "  val ue2 "
+bar3 = "  val ue3 "
+
+`)
+	})
 }
 
 func TestFile_SaveTo(t *testing.T) {

testdata/TestFile_WriteTo.golden

@@ -84,5 +84,5 @@ true                   = 2+3=5
 ADDRESS                = """404 road,
 NotFound, State, 50000"""
 two_lines              = how about continuation lines?
-lots_of_lines          = 1 2 3 4 
+lots_of_lines          = "1 2 3 4 "