fixes #71, ability to use known keys and certificate for MITM. Breaking change
doc: OnRequest function handler needs to return req and resp

The `OnRequest().DoFunc()` function is expecting a function returning
both a modified request and a response (optionally nil). Fix some
documentation strings to reflect that.
Merge pull request #77 from vincentbernat/doc/OnRequest

doc: OnRequest function handler needs to return req and resp
fixes #71, do not reuse tls.defaultTlsConfig
Merge remote-tracking branch 'upstream/master'

Conflicts:
	proxy_test.go