Provide (fully covered) integration tests for all client methods

[jsamoocha]

For effective CI and PR merging, all existing client endpoint tests from stravalib.tests.functional should be moved to or re-recreated in stravalib.tests.integration, and use the new mock_strava_api fixture to be independent of an actual connection to the Strava API. The tests in stravalib.tests.integration.test_client serve as examples.

For each test, let's try to cover all branching logic (and maybe use automated coverage tooling for that?)

The current swagger.json defines the following endpoints that need to be tested:

• /athletes/{id}/stats (GET)
• /athlete (GET)
• /athlete (PUT)
• /athlete/zones (GET)
• /segments/{id} (GET)
• /segments/starred (GET)
• /segments/{id}/starred (PUT) - NOT IMPLEMENTED AT ALL
• /segment_efforts (GET)
• /segments/explore (GET)
• /segment_efforts/{id} (GET)
• /activities (POST)
• /activities/{id} (GET)
• /activities/{id} (PUT)
• /athlete/activities (GET)
• /activities/{id}/laps (GET)
• /activities/{id}/zones (GET)
• /activities/{id}/comments (GET)
• /activities/{id}/kudos (GET)
• /clubs/{id} (GET)
• /clubs/{id}/members (GET)
• /clubs/{id}/admins (GET)
• /clubs/{id}/activities (GET)
• /athlete/clubs (GET)
• /gear/{id} (GET)
• /routes/{id} (GET)
• /athletes/{id}/routes (GET)
• /routes/{id}/export_gpx (GET)
• /routes/{id}/export_tcx (GET)
• /uploads (POST)
• /uploads/{uploadId} (GET)
• /activities/{id}/streams (GET)
• /segment_efforts/{id}/streams (GET)
• /segments/{id}/streams (GET)
• /routes/{id}/streams (GET)

This is quite a lot of work, so let's gradually add these tests in small PRs and check them off the list above.

[jsamoocha]

#256 takes care of /uploads, /activities{id} (GET), and /athlete (GET).

[jsamoocha]

Oops, I forgot to take into account multiple HTTP methods per endpoint, so added these to the checklist.