Fix #216: HttpMessage._add_default_headers does not overwrite existing headers

[asvetlov]

I have had drop SERVER and DATE from hop-by-hop headers also.
According to http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html 13.5.1 End-to-end and Hop-by-hop Headers those headers are not in hop-by-hop list.

[fafhrd91]

+1 for useragent

i'm not sure about server and date, here is reason:

# Server and Date aren't technically hop-by-hop
# headers, but they are in the purview of the
# origin server which the WSGI spec says we should
# act like. So we drop them and add our own.

[fafhrd91]

+1 for server and date

[kxepal]

+1

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