1.7 write-through proxy to 1.6 master

It's possible to setup a 1.7 write-through proxy to a 1.6 master. This
works provided that the 1.7 proxy uses "SVNAdvertiseV2Protocol off",
but if the proxy doesn't disable V2 it forwards V2 requests to the
V1 master which results in ugly errors:

svn: E175002: Commit failed (details follow):
svn: E175002: Server sent unexpected return value (500 Internal Server Error) in response to POST request for '/repos/asf/!svn/me'

I'm not familiar with the proxy code. I think a V2 proxy should check
whether the master advertises V2 support and return a more intelligible
error if it does not. I suppose it only needs to do this if the client
is using V2 write commands, as a client using V1, or V2 read-only, will
work fine. I suspect it is not possible for the proxy to switch to V1
automatically since I think the V2 support negotiated before the proxy
contacts the master.

Perhaps this is a valid use for the mythical "use V1" client switch :)

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