DeArmor: remove whitespace of blank line

RFC on Forming ASCII Armor:

> - A blank (zero-length, or containing only whitespace) line

This blank line between header and data can currently not contain whitespaces as there are several .split('\n\n') in openpgp.encoding.asciiarmor.js

As a result we cannot import this key:
```
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v2.0.20 (MingW32)
 
mQENBFGvhyEBCADCaTM6fAoQJSfjKqKM2fzm6vdjZ88T85RIhr3s/rdt8e/NrmIx
SdmnCXj5NFRnIUw+RZcKXLyLzGE970kl+yJPiFjqp2So6hTldw68zz5f1PL8vGTB
YyaHg4RWpnqtT2leUbA7rfl21TyNCIs0s9j8Eqjn0+WFaJnLfUCVmX+U7yj32idu
qh7/nnSaAMhdhFUqIaVLJKMyv9BuGdw5/nv1WQEKZ/SnkrBMbZFIQvXrXpCjagKO
Mgn7EpNwDHAFGp8YfyxyMyX4fPgDCcFHqlB6vE9ZJm6kztHDlFXv6VFbKVw6m+OB
8d4lcUv3DgqFP3ZgfRYEcvoaf5VrQQ3y0XFVABEBAAG0IlTDs3RoIEF0dGlsYSA8
YXR0aWxhLnRvdGhAZW5ldC5odT6JATkEEwECACMFAlGvhyECGwMHCwkIBwMCAQYV
CAIJCgsEFgIDAQIeAQIXgAAKCRBd7Wviv/dV3m0OB/wJbos8VqECoXty0aznpPof
LJFO0r9qw5WzChtMHk7l5uQNxHEoAlPCDuotrD5jocnr2nm92h2qB6hHy+ZO0vOj
+XNoSJOJDdiprWDFE03xecNcKmKizDvvdoyYmuFzd/6DdJaah4wo2RrdlGUZ6x5X
RJulmqEizkUEXv3zJxayFFBjP49UuceOxUQ/sY2qEXE6N13QMKBOC9eF5BJTFmK5
43A551xZ23uu8IfTxGXRq2C6LQyKk4qChESbUsx16Ck5SjGglOnzI+0o587a+/wa
/Ylu7h5vN1Zeq9Sa3kY3mUIMTrZdJDjcvKZ5cjo34GSm+NQ6RWq+WNUIQ7s0slmx
uQENBFGvhyEBCADTcgxuC8Bj40OgUx73d82QU6/S2ld3tGNYYIKNDztgk/sd7Oj3
MEvs46/Tr6FGHkBtSzQpRU4wpwXTKLONonVe8SMBHHZFIwrlObjt/WtultsASM3i
z4hj9+3vXX+Tm3xktlDtLCWh8jD0yANGMfUrHMESd9ydY2+AxtqhGMlhdXBZITsJ
T4ejuhZ7QwTwBMJRN6qXDJPS0dMfR7+Tlawlzm2PfPFhrdF5GxhJbPm9sIDyZpo2
p4oVzy/nNyTjIKobRDST3vZ8W0AhhVbTW6hq7jQRkAKIJ+yZxqfLMvVbz9qUMnwi
VX6iDfXPSGrqg7/xkbaHa4HUGGliSilm1Ai/ABEBAAGJAR4EGAECAAkFAlGvhyEC
GwwACgkQXe1r4r/3Vd5eFAf3W2tUAMbVOi6YtDmEAuECUubdZfHO3LG6c/xlhmIJ
scTM3wxrVrE7EjnMUsYWPg43QnstQJfb7W9SlLKpvtmn29zKpa2t+WSO7+GahCvm
x8tKfLwvPoCoM1ZQy0U7wBwxGlr+St5v5nrCxDQ8Eee3WriYiaaMba+/B+9QIXoe
+jTilmxU6t8WY2fachwzmiiQaywaZx9sK4XrS7o5xFzsbvEnOsRi3ipRlXl4fgPt
DqdL7K4LIVr0aQouHhnL0fZkBTsTrNZJdqwuytjZbG90cs7OA2W/F+9BYCXvY1RJ
2oAUJCYMTHuhLp1dGKSSc+TMnzbhv1SpkN+z1cI4Aa7L
=4oNi
-----END PGP PUBLIC KEY BLOCK-----
```