encoding/pem: do not try to round trip value with leading/trailing space

The header is literally

	Key: Value

If the value or the key has leading or trailing spaces, those will
be lost by the round trip.

Found because testing/quick returns different values now.

Change-Id: I0f574bdbb5990689509c24309854d8f814b5efa0
Reviewed-on: https://go-review.googlesource.com/39211
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/pem/pem_test.go

@@ -207,9 +207,10 @@ func TestLineBreaker(t *testing.T) {
 
 func TestFuzz(t *testing.T) {
 	testRoundtrip := func(block Block) bool {
-		for key := range block.Headers {
-			if strings.Contains(key, ":") {
-				// Keys with colons cannot be encoded.
+		for key, val := range block.Headers {
+			if strings.ContainsAny(key, ":\r\n") || strings.ContainsAny(val, "\r\n") || strings.TrimSpace(key) != key || strings.TrimSpace(val) != val {
+				// Keys with colons or newlines cannot be encoded.
+				// Keys/values with surrounding spaces might lose theirs.
 				return true
 			}
 		}