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

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>