[Openid-specs-fapi] Issue #40: 2nd WD: Part 1: 6.2.1: Constraint on the InteractionId string uncliear (openid/fapi)

[Nat Sakimura]

New issue 40: 2nd WD: Part 1: 6.2.1: Constraint on the InteractionId string uncliear
https://bitbucket.org/openid/fapi/issues/40/2nd-wd-part-1-621-constraint-on-the

Nat Sakimura:

`x-fapi-InteractionId` , which used to be `DDA-InteractionId` looks like a string but the constraint is not stated. We should state it. 

Current example: 

    x-fapi-InteractionId: c770aef3-6784-41f7-8e0e-ff5f97bddb3a