ensure plugin object is defined when dereferencing payloadHash

It is possible (though not valid) for a request to contain
a hash in the Authorization header but no payload in the
request body. When this happens plugins['hapi-auth-hawk']
will not be defined, so dereferencing payloadHash throws
an error. This patch ensure payloadHash can always be
dereferenced.
added test case for absent payload with validation
Merge pull request #6 from dannycoates/i2

Fixed an uncaught error when payload validation is enabled but payload is absent