Support setting multiple request headers with same name

[svanoort]

Currently headers on requests are parsed and stored into a dictionary of key, value pairs.

It would be nice to convert that to a list of (key,value) tuples to support setting duplicate headers (example cookie values).

This requires changing the tests & test_tests classes, changing how headers are parsed, stored and passed to pycurl. Which sounds like a lot but is, in fact, super-duper easy.

if head: #Convert headers dictionary to list of header entries, tested and working
            headers = [str(headername)+':'+str(headervalue) for headername, headervalue in head.items()]
# BECOMES
if head: #Convert headers dictionary to list of header entries, tested and working
            headers = [str(headername)+':'+str(headervalue) for (headername, headervalue) in head]

Note that for back-compatibility, we need to support still receiving a yaml dictionary of header values for test config.

[svanoort]

Blocked by the use of flatten_dicts in the test parsing logic. sigh. Depends on parsing refactors in: #75