CR123: Simultaneous presence of {http method} and {safe}

I've implemented the current resolution for CR123.

However, I think we need to do a little more. Indeed, currently:
- if both {http method} (or, in its absence, {http method default}) and 
{safe} are set, {http method} takes precendence, i.e. the value of the 
{safe} property is ignored.

This could be an issue, for example if {http method}="POST" and 
{safe}="true".

Two alternatives:
a. Indicate {http method} and {safe} cannot be present simultaneously.

b. {safe} takes precendence over {http method} (as {http method} already 
takes precendence over {http method default}).

There's some appealing symmetry in b., but a. may be straigther.

JJ.