Change SHALL to MAY for standard properties

[Zarquan]

The introductory text for section 3.2.4 Standard properties uses the word SHOULD to indicate that the use of these properties is recommended but not mandatory.

The following URIs SHOULD be used to represent the service properties:

However the text for the individual properties used the word SHALL, which suggests that the use of these properties is a mandatory requirement of the specification.

ivo://ivoa.net/vospace/core#title SHALL be used as the property URI denoting a name given to the resource

We should reserve the mandatory keywords MUST and SHALL for the parts of the specification that are essential for the protocol to function. The use of these common properties aids interoperability, but services and clients should be free to use different identifiers for these concepts if they need to.

See Key words for use in RFCs to Indicate Requirement Levels RFC 2119

[zonia3000]

Pull request #20 is opened for this.