Add unit tests for file endpoints

[pavog]

This PR adds unit tests for the file endpoints of CurseForgeAPIClient. They run against the mocked HTTP client from ClientTestCase.

getModFiles():

• With no options, only index=0 and pageSize=50 are sent. The results are mapped to File.
• All options are set and every query parameter is checked. clientCompatible is sent as true (see Fix boolean query parameters #14).
• getNextPage() keeps the filters of the first request and only changes the offset.

getModFile()

• getFiles(): sends a POST to /v1/mods/files with fileIds in the body
• getModFileChangelog(): once without query parameters, once with raw, stripped and markup
• getModFileDownloadURL()

The fixtures are recorded responses from the real CurseForge API for mclogs (420561).