ACTION-679: Propose text for para 2 of 3.1.1

Here's my proposal for mildly strengthening the wording about how CT proxies should behave if they cannot be sure that the user agent is a browser. 

Current text:   Irrespective of the presence of the no-transform directive, the proxy must behave transparently (q.v.) if it detects that the user agent is not a browser. 


Proposed text:  Irrespective of the presence of the no-transform directive, the proxy must behave transparently (q.v.) unless it is able to determine positively that the user agent is a browser.   The mechanism by which the proxy recognizes the user agent as a browser should  use evidence from the HTTP request, in particular the  user-agent and accept headers. 
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Martin