Proxy request headers

[thoughtarray]

The app already passes headers that are defined when testapp is started (-H/--header cli args) during chain-script requests. Request headers should also be passed.

Example:

testapp -n foo -p 5050 -d 'localhost:5000/' -H 'X-Route={}'

curl -H 'X-Foo: baz' 'localhost:5050?chain=bar'

# foo should send two headers to localhost:5050 (may be a proxy):
#   X-Route: bar
#   X-Foo: baz