fix(carto): Resolve conflicts between base URL and parameters

[donmccurdy]

Changes:

• If requestWithParameters base URL conflicts with given parameters, override base URL
• Use URLSearchParams API to merge non-conflicting parameters cleanly
• Move DEFAULT_PARAMETERS and DEFAULT_HEADERS to local constants in request-with-parameters.ts. When adding new DEFAULT_PARAMETERS we need to be careful of the precedence (user params > default params > base url params) and that information is clearer in context.
• Update unit tests, since URLSearchParams#toString handles whitespace and trailing slashes differently than the previous URL building logic.

[coveralls]

coverage: 89.896% (+0.006%) from 89.89%
when pulling 1e75ad7 on donmccurdy/fix-requestwithparams-override-baseurl
into 1e8b750 on master.