net/textproto: fix CanonicalMIMEHeaderKey panic

Fixes #6712

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/21450043

src/pkg/net/textproto/reader.go

@@ -574,13 +574,10 @@ func canonicalMIMEHeaderKey(a []byte) string {
 		// and upper case after each dash.
 		// (Host, User-Agent, If-Modified-Since).
 		// MIME headers are ASCII only, so no Unicode issues.
-		if a[i] == ' ' {
-			a[i] = '-'
-			upper = true
-			continue
-		}
 		c := a[i]
-		if upper && 'a' <= c && c <= 'z' {
+		if c == ' ' {
+			c = '-'
+		} else if upper && 'a' <= c && c <= 'z' {
 			c -= toLower
 		} else if !upper && 'A' <= c && c <= 'Z' {
 			c += toLower

src/pkg/net/textproto/reader_test.go

@@ -25,6 +25,10 @@ var canonicalHeaderKeyTests = []canonicalHeaderKeyTest{
 	{"user-agent", "User-Agent"},
 	{"USER-AGENT", "User-Agent"},
 	{"üser-agenT", "üser-Agent"}, // non-ASCII unchanged
+
+	// This caused a panic due to mishandling of a space:
+	{"C Ontent-Transfer-Encoding", "C-Ontent-Transfer-Encoding"},
+	{"foo bar", "Foo-Bar"},
 }
 
 func TestCanonicalMIMEHeaderKey(t *testing.T) {