tests the exercise the core functions

[dwinter]

We need tests.

For a start, let's aim to exercise all the functions for

• Creating queries for httr
• Making body's for httr POSTS
• Setting content headers
• GETing URLS
• POSTing to URLS
• returned parsed xml/json

In addition, we need to

• Test examples from the README / vignette
• Include covr line coverage (althought it will be painful to read!)