Fix float64 truncation error (#293)

Don't truncate float64 representation on marashaling.

Fixes https://github.com/pelletier/go-toml/issues/289

marshal_OrderPreserve_test.toml

@@ -27,6 +27,7 @@ title = "TOML Marshal Testing"
   uint = 5001
   bool = true
   float = 123.4
+  float64 = 123.456782132399
   int = 5000
   string = "Bite me"
   date = 1979-05-27T07:32:00Z

marshal_test.go

@@ -135,7 +135,8 @@ type testMapDoc struct {
 type testDocBasics struct {
 	Uint       uint      `toml:"uint"`
 	Bool       bool      `toml:"bool"`
-	Float      float32   `toml:"float"`
+	Float32    float32   `toml:"float"`
+	Float64    float64   `toml:"float64"`
 	Int        int       `toml:"int"`
 	String     *string   `toml:"string"`
 	Date       time.Time `toml:"date"`
@@ -174,7 +175,8 @@ var docData = testDoc{
 	Basics: testDocBasics{
 		Bool:       true,
 		Date:       time.Date(1979, 5, 27, 7, 32, 0, 0, time.UTC),
-		Float:      123.4,
+		Float32:    123.4,
+		Float64:    123.456782132399,
 		Int:        5000,
 		Uint:       5001,
 		String:     &biteMe,

marshal_test.toml

@@ -4,6 +4,7 @@ title = "TOML Marshal Testing"
   bool = true
   date = 1979-05-27T07:32:00Z
   float = 123.4
+  float64 = 123.456782132399
   int = 5000
   string = "Bite me"
   uint = 5001

tomltree_write.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"math"
+	"math/big"
 	"reflect"
 	"sort"
 	"strconv"
@@ -106,12 +107,20 @@ func tomlValueStringRepresentation(v interface{}, indent string, arraysOneElemen
 	case int64:
 		return strconv.FormatInt(value, 10), nil
 	case float64:
-		// Ensure a round float does contain a decimal point. Otherwise feeding
-		// the output back to the parser would convert to an integer.
-		if math.Trunc(value) == value {
-			return strings.ToLower(strconv.FormatFloat(value, 'f', 1, 32)), nil
+		// Default bit length is full 64
+		bits := 64
+		// Float panics if nan is used
+		if !math.IsNaN(value) {
+			// if 32 bit accuracy is enough to exactly show, use 32
+			_, acc := big.NewFloat(value).Float32()
+			if acc == big.Exact {
+				bits = 32
+			}
 		}
-		return strings.ToLower(strconv.FormatFloat(value, 'f', -1, 32)), nil
+		if math.Trunc(value) == value {
+			return strings.ToLower(strconv.FormatFloat(value, 'f', 1, bits)), nil
+		}
+		return strings.ToLower(strconv.FormatFloat(value, 'f', -1, bits)), nil
 	case string:
 		if tv.multiline {
 			return "\"\"\"\n" + encodeMultilineTomlString(value) + "\"\"\"", nil