[Seaside] Better protocol conformity

[Nevin Pratt]

It would be nice to get a little more protocol conformity between 
Comanche's "HttpRequest" class and Seaside's "IARequest" class. 
 Likewise for the "HttpResponse" and "IAResponse" classes.

For example, let's say your code wants to read the host from the 
request.  You can get the host from an HttpRequest via

     request header at: 'host'

However, to do the same with an "IARequest" instance, you do this:

     request headerAt: 'host'
           or possibly this...
     request headers at: 'host'

Just adding the method #header to the IARequest class so that it is the 
same as the current #headers method would solve this little incompatibility.

For that matter, it would be nice to completely ditch the IARequest and 
IAResponse classes and just use Comanche's.  But, lacking that, at least 
get better protocol conformity between them.

Nevin