Avoid extra quotes where not strictly necessary.

It's not necessary to enclose in quotes every single string value
in log2met format; when using basic words, it's possible to not
quote it (as heroku does for its own logging). This keeps the
logs easier on the human eye.

text_formatter.go

@@ -3,6 +3,7 @@ package logrus
 import (
 	"bytes"
 	"fmt"
+	"regexp"
 	"sort"
 	"strings"
 	"time"
@@ -19,11 +20,13 @@ const (
 var (
 	baseTimestamp time.Time
 	isTerminal    bool
+	noQuoteNeeded *regexp.Regexp
 )
 
 func init() {
 	baseTimestamp = time.Now()
 	isTerminal = IsTerminal()
+	noQuoteNeeded, _ = regexp.Compile("^[a-zA-Z0-9.-]*$")
 }
 
 func miniTS() int {
@@ -87,8 +90,18 @@ func printColored(b *bytes.Buffer, entry *Entry, keys []string) {
 
 func (f *TextFormatter) appendKeyValue(b *bytes.Buffer, key, value interface{}) {
 	switch value.(type) {
-	case string, error:
+	case string:
+		if noQuoteNeeded.MatchString(value.(string)) {
+			fmt.Fprintf(b, "%v=%s ", key, value)
+		} else {
 			fmt.Fprintf(b, "%v=%q ", key, value)
+		}
+	case error:
+		if noQuoteNeeded.MatchString(value.(error).Error()) {
+			fmt.Fprintf(b, "%v=%s ", key, value)
+		} else {
+			fmt.Fprintf(b, "%v=%q ", key, value)
+		}
 	default:
 		fmt.Fprintf(b, "%v=%v ", key, value)
 	}

text_formatter_test.go

@@ -0,0 +1,33 @@
+package logrus
+
+import (
+	"bytes"
+	"errors"
+
+	"testing"
+)
+
+func TestQuoting(t *testing.T) {
+	tf := new(TextFormatter)
+
+	checkQuoting := func(q bool, value interface{}) {
+		b, _ := tf.Format(WithField("test", value))
+		idx := bytes.LastIndex(b, []byte{'='})
+		cont := bytes.Contains(b[idx:], []byte{'"'})
+		if cont != q {
+			if q {
+				t.Errorf("quoting expected for: %#v", value)
+			} else {
+				t.Errorf("quoting not expected for: %#v", value)
+			}
+		}
+	}
+
+	checkQuoting(false, "abcd")
+	checkQuoting(false, "v1.0")
+	checkQuoting(true, "/foobar")
+	checkQuoting(true, "x y")
+	checkQuoting(true, "x,y")
+	checkQuoting(false, errors.New("invalid"))
+	checkQuoting(true, errors.New("invalid argument"))
+}