Optimize Sugar logger for calls with a single string arg (uber-go#913)

Currently, the Sugar logger uses fmt.Sprint in all cases when the
template is empty. However, this call is unnecessary if there's
a single string type argument, as we can use it directly.

With this optimization, we reduce the cost and avoid an unnecessary
alloc:
```
> benchcmp pre post
benchmark                         old ns/op     new ns/op     delta
BenchmarkSugarSingleStrArg-10     636           570           -10.38%

benchmark                         old allocs     new allocs     delta
BenchmarkSugarSingleStrArg-10     1              0              -100.00%
```

sugar.go

@@ -222,19 +222,30 @@ func (s *SugaredLogger) log(lvl zapcore.Level, template string, fmtArgs []interf
 		return
 	}
 
-	// Format with Sprint, Sprintf, or neither.
-	msg := template
-	if msg == "" && len(fmtArgs) > 0 {
-		msg = fmt.Sprint(fmtArgs...)
-	} else if msg != "" && len(fmtArgs) > 0 {
-		msg = fmt.Sprintf(template, fmtArgs...)
-	}
-
+	msg := getMessage(template, fmtArgs)
 	if ce := s.base.Check(lvl, msg); ce != nil {
 		ce.Write(s.sweetenFields(context)...)
 	}
 }
 
+// getMessage format with Sprint, Sprintf, or neither.
+func getMessage(template string, fmtArgs []interface{}) string {
+	if len(fmtArgs) == 0 {
+		return template
+	}
+
+	if template != "" {
+		return fmt.Sprintf(template, fmtArgs...)
+	}
+
+	if len(fmtArgs) == 1 {
+		if str, ok := fmtArgs[0].(string); ok {
+			return str
+		}
+	}
+	return fmt.Sprint(fmtArgs...)
+}
+
 func (s *SugaredLogger) sweetenFields(args []interface{}) []Field {
 	if len(args) == 0 {
 		return nil

sugar_test.go

@@ -368,3 +368,11 @@ func TestSugarAddCallerFail(t *testing.T) {
 			"Expected original message to survive failures in runtime.Caller.")
 	})
 }
+
+func BenchmarkSugarSingleStrArg(b *testing.B) {
+	withSugar(b, InfoLevel, nil /* opts* */, func(log *SugaredLogger, logs *observer.ObservedLogs) {
+		for i := 0; i < b.N; i++ {
+			log.Info("hello world")
+		}
+	})
+}