Performance improvements.

BenchmarkResponseWriter     1239          1304          +5.25%
BenchmarkFullSSE            1469          915           -37.71%
BenchmarkNoRetrySSE         1187          785           -33.87%
BenchmarkSimpleSSE          961           687           -28.51%

benchmark                   old allocs     new allocs     delta
BenchmarkResponseWriter     9              9              +0.00%
BenchmarkFullSSE            12             3              -75.00%
BenchmarkNoRetrySSE         11             2              -81.82%
BenchmarkSimpleSSE          8              2              -75.00%

benchmark                   old bytes     new bytes     delta
BenchmarkResponseWriter     442           442           +0.00%
BenchmarkFullSSE            462           320           -30.74%
BenchmarkNoRetrySSE         473           337           -28.75%
BenchmarkSimpleSSE          426           314           -26.29%

writer.go

@@ -0,0 +1,24 @@
+package sse
+
+import "io"
+
+type stringWriter interface {
+	io.Writer
+	WriteString(string) (int, error)
+}
+
+type stringWrapper struct {
+	io.Writer
+}
+
+func (w stringWrapper) WriteString(str string) (int, error) {
+	return w.Writer.Write([]byte(str))
+}
+
+func checkWriter(writer io.Writer) stringWriter {
+	if w, ok := writer.(stringWriter); ok {
+		return w
+	} else {
+		return stringWrapper{writer}
+	}
+}