Fix unmarshalling of OriginCACertificate

[player-two]

Fixes #190

Description

The format of the expires_on field differs from what is documented, and the json parser makes CreateOriginCertificate() always return an error, even if the certificate was created. This change introduces custom timestamp parsing that works for both the actual and documented formats.

Has your change been tested?

I wrote a tiny test program that calls CreateOriginCertificate() and checked that the cert object is populated and the error is nil.

Screenshots (if appropriate):

N/A

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[player-two]

The other way to fix this is introduce a new type for just the field and modify the struct definition to

type OriginCACertificate struct {
	ExpiresOn certificateExpiresOn `json:"expires_on,omitempty"`

That would remove the need to overload MarshalJSON() for the struct, but it would mean any code accessing the field would have to explicitly convert to time.Time. I opted to introduce a little more mess here to make usage easier.

[patryk]

Thanks @player-two ! I have modified it slightly with help of the aliasing method described in http://choly.ca/post/go-json-marshalling/