Allow starting dummyserver in standalone mode

I didn't find a way to launch dummyserver stand-alone. Helps writing tests
if you can launch the dummyserver and just shoot at it with HTTPie.

$ python -m dummyserver.server
Merge pull request #503 from joneskoo/dummyserver-standalone

Allow starting dummyserver in standalone mode.
Add failing test for #459

Reproduces the original issue.

When I upload with requests.put(...) an open file
handle and simultaneously enable retries, urllib3
fails as it tries to retry upload without seeking
the file object.

Critically content-length needs to be set to
reproduce failure; otherwise data corruption will
occur as only partial content will be uploaded!

Thanks to @sigmavirus24 for continued support
with this issue.
Add a test variant without content length

Just to show what happens when content length is not set.
File is not seeked => second upload fails with content length
0 as the initial attempt consumed the whole file.