Fix #68: missing lines in signature string example.

[ioggstream]

This PR

Adds missing virtual headers in signature string: (created) (expires).

Fixes: #68

[hrobache]

Actually, it seems there is an inconsistency within C.3. All headers test.

1. the signing string is
   (request-target): post /foo?param=value&pet=dog
   host: example.com
   date: Sun, 05 Jan 2014 21:31:40 GMT
   content-type: application/json
   digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=
   content-length: 18

2. the authorization header is
   Authorization: Signature keyId="Test",algorithm="rsa-sha256",
   created=1402170695, expires=1402170699,
   headers="(request-target) (created) (expires) host date content-type digest content-length",
   signature="vSdrb+dS3EceC9bcwHSo4MlyKS59iFIrhgYkz8+oVLEEzmYZZvRs
   8rgOp+63LEM3v+MFHB32NfpB2bEKBIvB1q52LaEUHFv120V01IL+TAD48XaERZF
   ukWgHoBTLMhYS2Gb51gWxpeIq8knRmPnYePbF5MOkR0Zkly4zKH7s1dE="

So:

• either add (created) and (expires) in the signing string and recompute the signature
• or remove (created) and (expires) from the "headers" part of the authorization header.

[ioggstream]

@hrobache Right. After adding created,expires I have to regenerate the signature value too. I'll try to do it soon