Do not insert a newline as the first character of a header

It is closer to Gmail behavior for example.
See #53

message_test.go

@@ -7,7 +7,6 @@ import (
 	"io/ioutil"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -683,32 +682,6 @@ func mockCopyFileWithHeader(m *Message, name string, h map[string][]string) (str
 	return name, f, SetHeader(h)
 }
 
-func TestLineLength(t *testing.T) {
-	switch runtime.Version()[:5] {
-	case "go1.2", "go1.3", "go1.4", "go1.5":
-		t.Skip("Only pass with Go 1.6+")
-	}
-
-	m := NewMessage()
-	m.SetAddressHeader("From", "from@example.com", "Señor From")
-	m.SetHeader("Subject", "{$firstname} Bienvendio a Apostólica, aquí inicia el camino de tu")
-	m.SetBody("text/plain", strings.Repeat("a", 100))
-
-	buf := new(bytes.Buffer)
-	m.WriteTo(buf)
-	n := 0
-	for _, b := range buf.Bytes() {
-		if b == '\n' {
-			n = 0
-		} else {
-			n++
-			if n == 80 {
-				t.Errorf("A line is too long:\n%s", buf.Bytes())
-			}
-		}
-	}
-}
-
 func BenchmarkFull(b *testing.B) {
 	discardFunc := SendFunc(func(from string, to []string, m io.WriterTo) error {
 		_, err := m.WriteTo(ioutil.Discard)

writeto.go

@@ -219,13 +219,6 @@ func (w *messageWriter) writeLine(s string, charsLeft int) string {
 		}
 	}
 
-	// No space found so insert a newline if it was not the start of the
-	// line.
-	if charsLeft != 75 {
-		w.writeString("\r\n ")
-		return s
-	}
-
 	// We could not insert a newline cleanly so look for a space or a newline
 	// even if it is after the limit.
 	for i := 75; i < len(s); i++ {