(SERVER-43) test coverage for ruby HTTP client

Refactor test for extensibility and add a test case for HTTP POST.
Use a ScriptingContainer with persistent local variables, so we can
re-use the HttpClient instance for additional tests.
(SERVER-43) use 'strict_encode64' and add tests

'encode64' incorrectly adds a '\n' to the end of the encoded string -
use 'strict_encode64' which isn't stupid.

Also remove the errant ':' character in the 'Authorization' header used
by the ruby HTTP client.