Refactor proxy tests

This commit introduces a macro to simplify the proxy tests; this
will allow us to write several more tests w/o duplicating all
of the proxy setup code.
Add support for HTTPS when proxying
Add support for specifying scheme and ssl config for proxy target

This commit adds some additional fields that you can specify
when configuring a proxy target; specifically, you may now
include a `:scheme` to force http or https (rather than using
the scheme of the original request).  You can also specify
an `:ssl-config` map, to specify the pem files to use for
the ssl context.