URL encode auth body

[doneill]

Proposed Changes

• URL encode the login form
• Add support for request body
  • Send nil in existing GET requests
• Add tests

Expectation:

• Authenticate a user against a site
• Confirm success and GET calls return as expected
• Run test

go test -v ./api/... -run TestAuthenticate
=== RUN   TestAuthenticate
=== RUN   TestAuthenticate/successful_response
=== RUN   TestAuthenticate/special_characters_are_form_encoded
=== RUN   TestAuthenticate/invalid_credentials
--- PASS: TestAuthenticate (0.00s)
    --- PASS: TestAuthenticate/successful_response (0.00s)
    --- PASS: TestAuthenticate/special_characters_are_form_encoded (0.00s)
    --- PASS: TestAuthenticate/invalid_credentials (0.00s)
PASS
ok      github.com/doneill/er-cli/api   (cached)