http: fix FileServer's default text content type

Fixes #1729

R=rsc, adg
CC=golang-dev
https://golang.org/cl/4443057

src/pkg/http/fs.go

@@ -143,7 +143,7 @@ func serveFile(w ResponseWriter, r *Request, name string, redirect bool) {
 			n, _ := io.ReadFull(f, buf[:])
 			b := buf[:n]
 			if isText(b) {
-				ctype = "text-plain; charset=utf-8"
+				ctype = "text/plain; charset=utf-8"
 			} else {
 				// generic binary
 				ctype = "application/octet-stream"

src/pkg/http/fs_test.go

@@ -104,7 +104,7 @@ func TestServeFileContentType(t *testing.T) {
 			t.Errorf("Content-Type mismatch: got %q, want %q", h, want)
 		}
 	}
-	get("text-plain; charset=utf-8")
+	get("text/plain; charset=utf-8")
 	override = true
 	get(ctype)
 }