Example: Unpack Base64 using 'm0*' rather than 'm*'

`unpack('m*')` ignores non-Base-64 characters; `unpack('m0*')` raises
an exception `ArgumentError: invalid base64` on encountering an
invalid character.

The former follows [RFC 2046 §6.8], "Any characters outside of the
base64 alphabet are to be ignored in base64-encoded data." That has
been superseded by [RFC 4648 §3.3] which points out that this
introduces security and other issues, and says "Implementations MUST
reject the encoded data if it contains characters outside the base
alphabet when interpreting base-encoded data, unless the specification
referring to this document explicitly states otherwise."

Potential security issues aside, it's clear that here it's better to
require a Base 64 string with no invalid characters so that the caller
finds it easier to diagnose errors in the parameters. We change it
here to provide a better example to other implementors of this idea
who may be copying the code.

There's a good argument to be made that we shouldn't be including the
"type" (i.e., `ssh-rsa` or whatever) at all at the start of the
hostkey parameter as it's not actually used by the underlying Net::SSH
code. This will be dealt with in a later commit.

[RFC 2046 §6.8]: https://tools.ietf.org/html/rfc2045#section-6.8
[RFC 4648 §3.3]: https://tools.ietf.org/html/rfc4648#section-3.3

Example.rb

@@ -151,7 +151,7 @@ def initialize(host, pubkeys)
         super(pubkeys.map { |keyline|
             type, key = keyline.split(' ', 2)
             # XXX we just assume it's a supported type, yeah, that's lazybad
-            blob = key.unpack('m*').first
+            blob = key.unpack('m0*').first
             Net::SSH::Buffer.new(blob).read_key
         })
     end